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More Language Features

Overview

With this last lecture, our advice is to skip it on first pass, unless you have a burning desire to read it

It’s here

  1. as a reference, so we can link back to it when required, and
  2. for those who have worked through a number of applications, and now want to learn more about the Python language

A variety of topics are treated in the lecture, including generators, exceptions and descriptors

Iterables and Iterators

We’ve already said something about iterating in Python

Now let’s look more closely at how it all works, focusing in Python’s implementation of the for loop

Iterators

Iterators are a uniform interface to stepping through elements in a collection

Here we’ll talk about using iterators—later we’ll learn how to build our own

Formally, an iterator is an object with a __next__ method

For example, file objects are iterators

To see this, let’s have another look at the US cities data, which is written to the present working directory in the following cell

In [1]:
%%file us_cities.txt
new york: 8244910
los angeles: 3819702
chicago: 2707120
houston: 2145146
philadelphia: 1536471
phoenix: 1469471
san antonio: 1359758
san diego: 1326179
dallas: 1223229
Overwriting us_cities.txt
In [2]:
f = open('us_cities.txt')
f.__next__()
Out[2]:
'new york: 8244910\n'
In [3]:
f.__next__()
Out[3]:
'los angeles: 3819702\n'

We see that file objects do indeed have a __next__ method, and that calling this method returns the next line in the file

The next method can also be accessed via the builtin function next(), which directly calls this method

In [4]:
next(f)
Out[4]:
'chicago: 2707120\n'

The objects returned by enumerate() are also iterators

In [5]:
e = enumerate(['foo', 'bar'])
next(e)
Out[5]:
(0, 'foo')
In [6]:
next(e)
Out[6]:
(1, 'bar')

as are the reader objects from the csv module

Let’s create a small csv file that contains data from the NIKKEI index

In [7]:
%%file test_table.csv
Date,Open,High,Low,Close,Volume,Adj Close
2009-05-21,9280.35,9286.35,9189.92,9264.15,133200,9264.15
2009-05-20,9372.72,9399.40,9311.61,9344.64,143200,9344.64
2009-05-19,9172.56,9326.75,9166.97,9290.29,167000,9290.29
2009-05-18,9167.05,9167.82,8997.74,9038.69,147800,9038.69
2009-05-15,9150.21,9272.08,9140.90,9265.02,172000,9265.02
2009-05-14,9212.30,9223.77,9052.41,9093.73,169400,9093.73
2009-05-13,9305.79,9379.47,9278.89,9340.49,176000,9340.49
2009-05-12,9358.25,9389.61,9298.61,9298.61,188400,9298.61
2009-05-11,9460.72,9503.91,9342.75,9451.98,230800,9451.98
2009-05-08,9351.40,9464.43,9349.57,9432.83,220200,9432.83
Writing test_table.csv
In [8]:
from csv import reader

f = open('test_table.csv', 'r')
nikkei_data = reader(f)
next(nikkei_data)
Out[8]:
['Date', 'Open', 'High', 'Low', 'Close', 'Volume', 'Adj Close']
In [9]:
next(nikkei_data)
Out[9]:
['2009-05-21', '9280.35', '9286.35', '9189.92', '9264.15', '133200', '9264.15']

Iterators in For Loops

All iterators can be placed to the right of the in keyword in for loop statements

In fact this is how the for loop works: If we write

for x in iterator:
    <code block>

then the interpreter

  • calls iterator.___next___() and binds x to the result
  • executes the code block
  • repeats until a StopIteration error occurs

So now you know how this magical looking syntax works

f = open('somefile.txt', 'r')
for line in f:
    # do something

The interpreter just keeps

  1. calling f.__next__() and binding line to the result
  2. executing the body of the loop

This continues until a StopIteration error occurs

Iterables

You already know that we can put a Python list to the right of in in a for loop

In [10]:
for i in ['spam', 'eggs']:
    print(i)
spam
eggs

So does that mean that a list is an iterator?

The answer is no

In [11]:
x = ['foo', 'bar']
type(x)
Out[11]:
list
In [12]:
next(x)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-12-92de4e9f6b1e> in <module>()
----> 1 next(x)

TypeError: 'list' object is not an iterator

So why can we iterate over a list in a for loop?

The reason is that a list is iterable (as opposed to an iterator)

Formally, an object is iterable if it can be converted to an iterator using the built-in function iter()

Lists are one such object

In [13]:
x = ['foo', 'bar']
type(x)
Out[13]:
list
In [14]:
y = iter(x)
type(y)
Out[14]:
list_iterator
In [15]:
next(y)
Out[15]:
'foo'
In [16]:
next(y)
Out[16]:
'bar'
In [17]:
next(y)
---------------------------------------------------------------------------
StopIteration                             Traceback (most recent call last)
<ipython-input-17-81b9d2f0f16a> in <module>()
----> 1 next(y)

StopIteration: 

Many other objects are iterable, such as dictionaries and tuples

Of course, not all objects are iterable

In [18]:
iter(42)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-18-ef50b48e4398> in <module>()
----> 1 iter(42)

TypeError: 'int' object is not iterable

To conclude our discussion of for loops

  • for loops work on either iterators or iterables
  • In the second case, the iterable is converted into an iterator before the loop starts

Iterators and built-ins

Some built-in functions that act on sequences also work with iterables

  • max(), min(), sum(), all(), any()

For example

In [19]:
x = [10, -10]
max(x)
Out[19]:
10
In [20]:
y = iter(x)
type(y)
Out[20]:
list_iterator
In [21]:
max(y)
Out[21]:
10

One thing to remember about iterators is that they are depleted by use

In [22]:
x = [10, -10]
y = iter(x)
max(y)
Out[22]:
10
In [23]:
max(y)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-23-062424e6ec08> in <module>()
----> 1 max(y)

ValueError: max() arg is an empty sequence

Names and Name Resolution

Variable Names in Python

Consider the Python statement

In [24]:
x = 42

We now know that when this statement is executed, Python creates an object of type int in your computer’s memory, containing

  • the value 42
  • some associated attributes

But what is x itself?

In Python, x is called a name, and the statement x = 42 binds the name x to the integer object we have just discussed

Under the hood, this process of binding names to objects is implemented as a dictionary—more about this in a moment

There is no problem binding two or more names to the one object, regardless of what that object is

In [25]:
def f(string):      # Create a function called f
    print(string)   # that prints any string it's passed

g = f
id(g) == id(f)
Out[25]:
True
In [26]:
g('test')
test

In the first step, a function object is created, and the name f is bound to it

After binding the name g to the same object, we can use it anywhere we would use f

What happens when the number of names bound to an object goes to zero?

Here’s an example of this situation, where the name x is first bound to one object and then rebound to another

In [27]:
x = 'foo'
id(x)
Out[27]:
140207328304408
In [28]:
x = 'bar'  # No names bound to the first object

What happens here is that the first object is garbage collected

In other words, the memory slot that stores that object is deallocated, and returned to the operating system

Namespaces

Recall from the preceding discussion that the statement

In [29]:
x = 42

binds the name x to the integer object on the right-hand side

We also mentioned that this process of binding x to the correct object is implemented as a dictionary

This dictionary is called a namespace

Definition: A namespace is a symbol table that maps names to objects in memory

Python uses multiple namespaces, creating them on the fly as necessary

For example, every time we import a module, Python creates a namespace for that module

To see this in action, suppose we write a script math2.py with a single line

In [30]:
%%file math2.py
pi = 'foobar'
Writing math2.py

Now we start the Python interpreter and import it

In [31]:
import math2

Next let’s import the math module from the standard library

In [32]:
import math

Both of these modules have an attribute called pi

In [33]:
math.pi
Out[33]:
3.141592653589793
In [34]:
math2.pi
Out[34]:
'foobar'

These two different bindings of pi exist in different namespaces, each one implemented as a dictionary

We can look at the dictionary directly, using module_name.__dict__

In [35]:
import math

math.__dict__.items()
Out[35]:
dict_items([('__name__', 'math'), ('__doc__', 'This module is always available.  It provides access to the\nmathematical functions defined by the C standard.'), ('__package__', ''), ('__loader__', <_frozen_importlib_external.ExtensionFileLoader object at 0x7f8492815438>), ('__spec__', ModuleSpec(name='math', loader=<_frozen_importlib_external.ExtensionFileLoader object at 0x7f8492815438>, origin='/home/quantecon/anaconda3/lib/python3.7/lib-dynload/math.cpython-37m-x86_64-linux-gnu.so')), ('acos', <built-in function acos>), ('acosh', <built-in function acosh>), ('asin', <built-in function asin>), ('asinh', <built-in function asinh>), ('atan', <built-in function atan>), ('atan2', <built-in function atan2>), ('atanh', <built-in function atanh>), ('ceil', <built-in function ceil>), ('copysign', <built-in function copysign>), ('cos', <built-in function cos>), ('cosh', <built-in function cosh>), ('degrees', <built-in function degrees>), ('erf', <built-in function erf>), ('erfc', <built-in function erfc>), ('exp', <built-in function exp>), ('expm1', <built-in function expm1>), ('fabs', <built-in function fabs>), ('factorial', <built-in function factorial>), ('floor', <built-in function floor>), ('fmod', <built-in function fmod>), ('frexp', <built-in function frexp>), ('fsum', <built-in function fsum>), ('gamma', <built-in function gamma>), ('gcd', <built-in function gcd>), ('hypot', <built-in function hypot>), ('isclose', <built-in function isclose>), ('isfinite', <built-in function isfinite>), ('isinf', <built-in function isinf>), ('isnan', <built-in function isnan>), ('ldexp', <built-in function ldexp>), ('lgamma', <built-in function lgamma>), ('log', <built-in function log>), ('log1p', <built-in function log1p>), ('log10', <built-in function log10>), ('log2', <built-in function log2>), ('modf', <built-in function modf>), ('pow', <built-in function pow>), ('radians', <built-in function radians>), ('remainder', <built-in function remainder>), ('sin', <built-in function sin>), ('sinh', <built-in function sinh>), ('sqrt', <built-in function sqrt>), ('tan', <built-in function tan>), ('tanh', <built-in function tanh>), ('trunc', <built-in function trunc>), ('pi', 3.141592653589793), ('e', 2.718281828459045), ('tau', 6.283185307179586), ('inf', inf), ('nan', nan), ('__file__', '/home/quantecon/anaconda3/lib/python3.7/lib-dynload/math.cpython-37m-x86_64-linux-gnu.so')])
In [36]:
import math2

math2.__dict__.items()
Out[36]:
dict_items([('__name__', 'math2'), ('__doc__', None), ('__package__', ''), ('__loader__', <_frozen_importlib_external.SourceFileLoader object at 0x7f848805a7b8>), ('__spec__', ModuleSpec(name='math2', loader=<_frozen_importlib_external.SourceFileLoader object at 0x7f848805a7b8>, origin='/home/quantecon/repos-collab/quantecon.build.lectures/_build_jupyter/py/math2.py')), ('__file__', '/home/quantecon/repos-collab/quantecon.build.lectures/_build_jupyter/py/math2.py'), ('__cached__', '/home/quantecon/repos-collab/quantecon.build.lectures/_build_jupyter/py/__pycache__/math2.cpython-37.pyc'), ('__builtins__', {'__name__': 'builtins', '__doc__': "Built-in functions, exceptions, and other objects.\n\nNoteworthy: None is the `nil' object; Ellipsis represents `...' in slices.", '__package__': '', '__loader__': <class '_frozen_importlib.BuiltinImporter'>, '__spec__': ModuleSpec(name='builtins', loader=<class '_frozen_importlib.BuiltinImporter'>), '__build_class__': <built-in function __build_class__>, '__import__': <built-in function __import__>, 'abs': <built-in function abs>, 'all': <built-in function all>, 'any': <built-in function any>, 'ascii': <built-in function ascii>, 'bin': <built-in function bin>, 'breakpoint': <built-in function breakpoint>, 'callable': <built-in function callable>, 'chr': <built-in function chr>, 'compile': <built-in function compile>, 'delattr': <built-in function delattr>, 'dir': <built-in function dir>, 'divmod': <built-in function divmod>, 'eval': <built-in function eval>, 'exec': <built-in function exec>, 'format': <built-in function format>, 'getattr': <built-in function getattr>, 'globals': <built-in function globals>, 'hasattr': <built-in function hasattr>, 'hash': <built-in function hash>, 'hex': <built-in function hex>, 'id': <built-in function id>, 'input': <bound method Kernel.raw_input of <ipykernel.ipkernel.IPythonKernel object at 0x7f848a4e4f60>>, 'isinstance': <built-in function isinstance>, 'issubclass': <built-in function issubclass>, 'iter': <built-in function iter>, 'len': <built-in function len>, 'locals': <built-in function locals>, 'max': <built-in function max>, 'min': <built-in function min>, 'next': <built-in function next>, 'oct': <built-in function oct>, 'ord': <built-in function ord>, 'pow': <built-in function pow>, 'print': <built-in function print>, 'repr': <built-in function repr>, 'round': <built-in function round>, 'setattr': <built-in function setattr>, 'sorted': <built-in function sorted>, 'sum': <built-in function sum>, 'vars': <built-in function vars>, 'None': None, 'Ellipsis': Ellipsis, 'NotImplemented': NotImplemented, 'False': False, 'True': True, 'bool': <class 'bool'>, 'memoryview': <class 'memoryview'>, 'bytearray': <class 'bytearray'>, 'bytes': <class 'bytes'>, 'classmethod': <class 'classmethod'>, 'complex': <class 'complex'>, 'dict': <class 'dict'>, 'enumerate': <class 'enumerate'>, 'filter': <class 'filter'>, 'float': <class 'float'>, 'frozenset': <class 'frozenset'>, 'property': <class 'property'>, 'int': <class 'int'>, 'list': <class 'list'>, 'map': <class 'map'>, 'object': <class 'object'>, 'range': <class 'range'>, 'reversed': <class 'reversed'>, 'set': <class 'set'>, 'slice': <class 'slice'>, 'staticmethod': <class 'staticmethod'>, 'str': <class 'str'>, 'super': <class 'super'>, 'tuple': <class 'tuple'>, 'type': <class 'type'>, 'zip': <class 'zip'>, '__debug__': True, 'BaseException': <class 'BaseException'>, 'Exception': <class 'Exception'>, 'TypeError': <class 'TypeError'>, 'StopAsyncIteration': <class 'StopAsyncIteration'>, 'StopIteration': <class 'StopIteration'>, 'GeneratorExit': <class 'GeneratorExit'>, 'SystemExit': <class 'SystemExit'>, 'KeyboardInterrupt': <class 'KeyboardInterrupt'>, 'ImportError': <class 'ImportError'>, 'ModuleNotFoundError': <class 'ModuleNotFoundError'>, 'OSError': <class 'OSError'>, 'EnvironmentError': <class 'OSError'>, 'IOError': <class 'OSError'>, 'EOFError': <class 'EOFError'>, 'RuntimeError': <class 'RuntimeError'>, 'RecursionError': <class 'RecursionError'>, 'NotImplementedError': <class 'NotImplementedError'>, 'NameError': <class 'NameError'>, 'UnboundLocalError': <class 'UnboundLocalError'>, 'AttributeError': <class 'AttributeError'>, 'SyntaxError': <class 'SyntaxError'>, 'IndentationError': <class 'IndentationError'>, 'TabError': <class 'TabError'>, 'LookupError': <class 'LookupError'>, 'IndexError': <class 'IndexError'>, 'KeyError': <class 'KeyError'>, 'ValueError': <class 'ValueError'>, 'UnicodeError': <class 'UnicodeError'>, 'UnicodeEncodeError': <class 'UnicodeEncodeError'>, 'UnicodeDecodeError': <class 'UnicodeDecodeError'>, 'UnicodeTranslateError': <class 'UnicodeTranslateError'>, 'AssertionError': <class 'AssertionError'>, 'ArithmeticError': <class 'ArithmeticError'>, 'FloatingPointError': <class 'FloatingPointError'>, 'OverflowError': <class 'OverflowError'>, 'ZeroDivisionError': <class 'ZeroDivisionError'>, 'SystemError': <class 'SystemError'>, 'ReferenceError': <class 'ReferenceError'>, 'MemoryError': <class 'MemoryError'>, 'BufferError': <class 'BufferError'>, 'Warning': <class 'Warning'>, 'UserWarning': <class 'UserWarning'>, 'DeprecationWarning': <class 'DeprecationWarning'>, 'PendingDeprecationWarning': <class 'PendingDeprecationWarning'>, 'SyntaxWarning': <class 'SyntaxWarning'>, 'RuntimeWarning': <class 'RuntimeWarning'>, 'FutureWarning': <class 'FutureWarning'>, 'ImportWarning': <class 'ImportWarning'>, 'UnicodeWarning': <class 'UnicodeWarning'>, 'BytesWarning': <class 'BytesWarning'>, 'ResourceWarning': <class 'ResourceWarning'>, 'ConnectionError': <class 'ConnectionError'>, 'BlockingIOError': <class 'BlockingIOError'>, 'BrokenPipeError': <class 'BrokenPipeError'>, 'ChildProcessError': <class 'ChildProcessError'>, 'ConnectionAbortedError': <class 'ConnectionAbortedError'>, 'ConnectionRefusedError': <class 'ConnectionRefusedError'>, 'ConnectionResetError': <class 'ConnectionResetError'>, 'FileExistsError': <class 'FileExistsError'>, 'FileNotFoundError': <class 'FileNotFoundError'>, 'IsADirectoryError': <class 'IsADirectoryError'>, 'NotADirectoryError': <class 'NotADirectoryError'>, 'InterruptedError': <class 'InterruptedError'>, 'PermissionError': <class 'PermissionError'>, 'ProcessLookupError': <class 'ProcessLookupError'>, 'TimeoutError': <class 'TimeoutError'>, 'open': <built-in function open>, 'copyright': Copyright (c) 2001-2018 Python Software Foundation.
All Rights Reserved.

Copyright (c) 2000 BeOpen.com.
All Rights Reserved.

Copyright (c) 1995-2001 Corporation for National Research Initiatives.
All Rights Reserved.

Copyright (c) 1991-1995 Stichting Mathematisch Centrum, Amsterdam.
All Rights Reserved., 'credits':     Thanks to CWI, CNRI, BeOpen.com, Zope Corporation and a cast of thousands
    for supporting Python development.  See www.python.org for more information., 'license': Type license() to see the full license text, 'help': Type help() for interactive help, or help(object) for help about object., '__IPYTHON__': True, 'display': <function display at 0x7f8491ee4048>, 'get_ipython': <bound method InteractiveShell.get_ipython of <ipykernel.zmqshell.ZMQInteractiveShell object at 0x7f8488428048>>}), ('pi', 'foobar')])

As you know, we access elements of the namespace using the dotted attribute notation

In [37]:
math.pi
Out[37]:
3.141592653589793

In fact this is entirely equivalent to math.__dict__['pi']

In [38]:
math.__dict__['pi'] == math.pi
Out[38]:
True

Viewing Namespaces

As we saw above, the math namespace can be printed by typing math.__dict__

Another way to see its contents is to type vars(math)

In [39]:
vars(math).items()
Out[39]:
dict_items([('__name__', 'math'), ('__doc__', 'This module is always available.  It provides access to the\nmathematical functions defined by the C standard.'), ('__package__', ''), ('__loader__', <_frozen_importlib_external.ExtensionFileLoader object at 0x7f8492815438>), ('__spec__', ModuleSpec(name='math', loader=<_frozen_importlib_external.ExtensionFileLoader object at 0x7f8492815438>, origin='/home/quantecon/anaconda3/lib/python3.7/lib-dynload/math.cpython-37m-x86_64-linux-gnu.so')), ('acos', <built-in function acos>), ('acosh', <built-in function acosh>), ('asin', <built-in function asin>), ('asinh', <built-in function asinh>), ('atan', <built-in function atan>), ('atan2', <built-in function atan2>), ('atanh', <built-in function atanh>), ('ceil', <built-in function ceil>), ('copysign', <built-in function copysign>), ('cos', <built-in function cos>), ('cosh', <built-in function cosh>), ('degrees', <built-in function degrees>), ('erf', <built-in function erf>), ('erfc', <built-in function erfc>), ('exp', <built-in function exp>), ('expm1', <built-in function expm1>), ('fabs', <built-in function fabs>), ('factorial', <built-in function factorial>), ('floor', <built-in function floor>), ('fmod', <built-in function fmod>), ('frexp', <built-in function frexp>), ('fsum', <built-in function fsum>), ('gamma', <built-in function gamma>), ('gcd', <built-in function gcd>), ('hypot', <built-in function hypot>), ('isclose', <built-in function isclose>), ('isfinite', <built-in function isfinite>), ('isinf', <built-in function isinf>), ('isnan', <built-in function isnan>), ('ldexp', <built-in function ldexp>), ('lgamma', <built-in function lgamma>), ('log', <built-in function log>), ('log1p', <built-in function log1p>), ('log10', <built-in function log10>), ('log2', <built-in function log2>), ('modf', <built-in function modf>), ('pow', <built-in function pow>), ('radians', <built-in function radians>), ('remainder', <built-in function remainder>), ('sin', <built-in function sin>), ('sinh', <built-in function sinh>), ('sqrt', <built-in function sqrt>), ('tan', <built-in function tan>), ('tanh', <built-in function tanh>), ('trunc', <built-in function trunc>), ('pi', 3.141592653589793), ('e', 2.718281828459045), ('tau', 6.283185307179586), ('inf', inf), ('nan', nan), ('__file__', '/home/quantecon/anaconda3/lib/python3.7/lib-dynload/math.cpython-37m-x86_64-linux-gnu.so')])

If you just want to see the names, you can type

In [40]:
dir(math)[0:10]
Out[40]:
['__doc__',
 '__file__',
 '__loader__',
 '__name__',
 '__package__',
 '__spec__',
 'acos',
 'acosh',
 'asin',
 'asinh']

Notice the special names __doc__ and __name__

These are initialized in the namespace when any module is imported

  • __doc__ is the doc string of the module
  • __name__ is the name of the module
In [41]:
print(math.__doc__)
This module is always available.  It provides access to the
mathematical functions defined by the C standard.
In [42]:
math.__name__
Out[42]:
'math'

Interactive Sessions

In Python, all code executed by the interpreter runs in some module

What about commands typed at the prompt?

These are also regarded as being executed within a module — in this case, a module called __main__

To check this, we can look at the current module name via the value of __name__ given at the prompt

In [43]:
print(__name__)
__main__

When we run a script using IPython’s run command, the contents of the file are executed as part of __main__ too

To see this, let’s create a file mod.py that prints its own __name__ attribute

In [44]:
%%file mod.py
print(__name__)
Writing mod.py

Now let’s look at two different ways of running it in IPython

In [45]:
import mod  # Standard import
mod
In [46]:
%run mod.py  # Run interactively
__main__

In the second case, the code is executed as part of __main__, so __name__ is equal to __main__

To see the contents of the namespace of __main__ we use vars() rather than vars(__main__)

If you do this in IPython, you will see a whole lot of variables that IPython needs, and has initialized when you started up your session

If you prefer to see only the variables you have initialized, use whos

In [47]:
x = 2
y = 3

import numpy as np

%whos
Variable      Type                          Data/Info
-----------------------------------------------------
e             enumerate                     <enumerate object at 0x7f84883e8f78>
f             function                      <function f at 0x7f84883350d0>
g             function                      <function f at 0x7f84883350d0>
i             str                           eggs
math          module                        <module 'math' from '/hom<...>37m-x86_64-linux-gnu.so'>
math2         module                        <module 'math2' from '/ho<...>ild_jupyter/py/math2.py'>
mod           module                        <module 'mod' from '/home<...>build_jupyter/py/mod.py'>
nikkei_data   reader                        <_csv.reader object at 0x7f84882e72e8>
np            module                        <module 'numpy' from '/ho<...>kages/numpy/__init__.py'>
reader        builtin_function_or_method    <built-in function reader>
x             int                           2
y             int                           3

The Global Namespace

Python documentation often makes reference to the “global namespace”

The global namespace is the namespace of the module currently being executed

For example, suppose that we start the interpreter and begin making assignments

We are now working in the module __main__, and hence the namespace for __main__ is the global namespace

Next, we import a module called amodule

import amodule

At this point, the interpreter creates a namespace for the module amodule and starts executing commands in the module

While this occurs, the namespace amodule.__dict__ is the global namespace

Once execution of the module finishes, the interpreter returns to the module from where the import statement was made

In this case it’s __main__, so the namespace of __main__ again becomes the global namespace

Local Namespaces

Important fact: When we call a function, the interpreter creates a local namespace for that function, and registers the variables in that namespace

The reason for this will be explained in just a moment

Variables in the local namespace are called local variables

After the function returns, the namespace is deallocated and lost

While the function is executing, we can view the contents of the local namespace with locals()

For example, consider

In [48]:
def f(x):
    a = 2
    print(locals())
    return a * x

Now let’s call the function

In [49]:
f(1)
{'x': 1, 'a': 2}
Out[49]:
2

You can see the local namespace of f before it is destroyed

The __builtins__ Namespace

We have been using various built-in functions, such as max(), dir(), str(), list(), len(), range(), type(), etc.

How does access to these names work?

  • These definitions are stored in a module called __builtin__
  • They have there own namespace called __builtins__
In [50]:
dir()[0:10]
Out[50]:
['In', 'Out', '_', '_11', '_13', '_14', '_15', '_16', '_19', '_2']
In [51]:
dir(__builtins__)[0:10]
Out[51]:
['ArithmeticError',
 'AssertionError',
 'AttributeError',
 'BaseException',
 'BlockingIOError',
 'BrokenPipeError',
 'BufferError',
 'BytesWarning',
 'ChildProcessError',
 'ConnectionAbortedError']

We can access elements of the namespace as follows

In [52]:
__builtins__.max
Out[52]:
<function max>

But __builtins__ is special, because we can always access them directly as well

In [53]:
max
Out[53]:
<function max>
In [54]:
__builtins__.max == max
Out[54]:
True

The next section explains how this works …

Name Resolution

Namespaces are great because they help us organize variable names

(Type import this at the prompt and look at the last item that’s printed)

However, we do need to understand how the Python interpreter works with multiple namespaces

At any point of execution, there are in fact at least two namespaces that can be accessed directly

(“Accessed directly” means without using a dot, as in pi rather than math.pi)

These namespaces are

  • The global namespace (of the module being executed)
  • The builtin namespace

If the interpreter is executing a function, then the directly accessible namespaces are

  • The local namespace of the function
  • The global namespace (of the module being executed)
  • The builtin namespace

Sometimes functions are defined within other functions, like so

In [55]:
def f():
    a = 2
    def g():
        b = 4
        print(a * b)
    g()

Here f is the enclosing function for g, and each function gets its own namespaces

Now we can give the rule for how namespace resolution works:

The order in which the interpreter searches for names is

  1. the local namespace (if it exists)
  2. the hierarchy of enclosing namespaces (if they exist)
  3. the global namespace
  4. the builtin namespace

If the name is not in any of these namespaces, the interpreter raises a NameError

This is called the LEGB rule (local, enclosing, global, builtin)

Here’s an example that helps to illustrate

Consider a script test.py that looks as follows

In [56]:
%%file test.py
def g(x):
    a = 1
    x = x + a
    return x

a = 0
y = g(10)
print("a = ", a, "y = ", y)
Writing test.py

What happens when we run this script?

In [57]:
%run test.py
a =  0 y =  11
In [58]:
x
Out[58]:
2

First,

  • The global namespace {} is created
  • The function object is created, and g is bound to it within the global namespace
  • The name a is bound to 0, again in the global namespace

Next g is called via y = g(10), leading to the following sequence of actions

  • The local namespace for the function is created
  • Local names x and a are bound, so that the local namespace becomes {'x': 10, 'a': 1}
  • Statement x = x + a uses the local a and local x to compute x + a, and binds local name x to the result
  • This value is returned, and y is bound to it in the global namespace
  • Local x and a are discarded (and the local namespace is deallocated)

Note that the global a was not affected by the local a

Mutable Versus Immutable Parameters

This is a good time to say a little more about mutable vs immutable objects

Consider the code segment

In [59]:
def f(x):
    x = x + 1
    return x

x = 1
print(f(x), x)
2 1

We now understand what will happen here: The code prints 2 as the value of f(x) and 1 as the value of x

First f and x are registered in the global namespace

The call f(x) creates a local namespace and adds x to it, bound to 1

Next, this local x is rebound to the new integer object 2, and this value is returned

None of this affects the global x

However, it’s a different story when we use a mutable data type such as a list

In [60]:
def f(x):
    x[0] = x[0] + 1
    return x

x = [1]
print(f(x), x)
[2] [2]

This prints [2] as the value of f(x) and same for x

Here’s what happens

  • f is registered as a function in the global namespace
  • x bound to [1] in the global namespace
  • The call f(x)

    • Creates a local namespace
    • Adds x to local namespace, bound to [1]
    • The list [1] is modified to [2]
    • Returns the list [2]
    • The local namespace is deallocated, and local x is lost
  • Global x has been modified

Handling Errors

Sometimes it’s possible to anticipate errors as we’re writing code

For example, the unbiased sample variance of sample $ y_1, \ldots, y_n $ is defined as

$$ s^2 := \frac{1}{n-1} \sum_{i=1}^n (y_i - \bar y)^2 \qquad \bar y = \text{ sample mean} $$

This can be calculated in NumPy using np.var

But if you were writing a function to handle such a calculation, you might anticipate a divide-by-zero error when the sample size is one

One possible action is to do nothing — the program will just crash, and spit out an error message

But sometimes it’s worth writing your code in a way that anticipates and deals with runtime errors that you think might arise

Why?

  • Because the debugging information provided by the interpreter is often less useful than the information on possible errors you have in your head when writing code
  • Because errors causing execution to stop are frustrating if you’re in the middle of a large computation
  • Because it’s reduces confidence in your code on the part of your users (if you are writing for others)

Assertions

A relatively easy way to handle checks is with the assert keyword

For example, pretend for a moment that the np.var function doesn’t exist and we need to write our own

In [61]:
def var(y):
    n = len(y)
    assert n > 1, 'Sample size must be greater than one.'
    return np.sum((y - y.mean())**2) / float(n-1)

If we run this with an array of length one, the program will terminate and print our error message

In [62]:
var([1])
---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-62-8419b6ab38ec> in <module>()
----> 1 var([1])

<ipython-input-61-e6ffb16a7098> in var(y)
      1 def var(y):
      2     n = len(y)
----> 3     assert n > 1, 'Sample size must be greater than one.'
      4     return np.sum((y - y.mean())**2) / float(n-1)

AssertionError: Sample size must be greater than one.

The advantage is that we can

  • fail early, as soon as we know there will be a problem
  • supply specific information on why a program is failing

Handling Errors During Runtime

The approach used above is a bit limited, because it always leads to termination

Sometimes we can handle errors more gracefully, by treating special cases

Let’s look at how this is done

Exceptions

Here’s an example of a common error type

In [63]:
def f:
  File "<ipython-input-63-262a7e387ba5>", line 1
    def f:
         ^
SyntaxError: invalid syntax

Since illegal syntax cannot be executed, a syntax error terminates execution of the program

Here’s a different kind of error, unrelated to syntax

In [64]:
1 / 0
---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
<ipython-input-64-bc757c3fda29> in <module>()
----> 1 1 / 0

ZeroDivisionError: division by zero

Here’s another

In [65]:
x1 = y1
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-65-a7b8d65e9e45> in <module>()
----> 1 x1 = y1

NameError: name 'y1' is not defined

And another

In [66]:
'foo' + 6
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-66-216809d6e6fe> in <module>()
----> 1 'foo' + 6

TypeError: can only concatenate str (not "int") to str

And another

In [67]:
X = []
x = X[0]
---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-67-082a18d7a0aa> in <module>()
      1 X = []
----> 2 x = X[0]

IndexError: list index out of range

On each occasion, the interpreter informs us of the error type

  • NameError, TypeError, IndexError, ZeroDivisionError, etc.

In Python, these errors are called exceptions

Catching Exceptions

We can catch and deal with exceptions using tryexcept blocks

Here’s a simple example

In [68]:
def f(x):
    try:
        return 1.0 / x
    except ZeroDivisionError:
        print('Error: division by zero.  Returned None')
    return None

When we call f we get the following output

In [69]:
f(2)
Out[69]:
0.5
In [70]:
f(0)
Error: division by zero.  Returned None
In [71]:
f(0.0)
Error: division by zero.  Returned None

The error is caught and execution of the program is not terminated

Note that other error types are not caught

If we are worried the user might pass in a string, we can catch that error too

In [72]:
def f(x):
    try:
        return 1.0 / x
    except ZeroDivisionError:
        print('Error: Division by zero.  Returned None')
    except TypeError:
        print('Error: Unsupported operation.  Returned None')
    return None

Here’s what happens

In [73]:
f(2)
Out[73]:
0.5
In [74]:
f(0)
Error: Division by zero.  Returned None
In [75]:
f('foo')
Error: Unsupported operation.  Returned None

If we feel lazy we can catch these errors together

In [76]:
def f(x):
    try:
        return 1.0 / x
    except (TypeError, ZeroDivisionError):
        print('Error: Unsupported operation.  Returned None')
    return None

Here’s what happens

In [77]:
f(2)
Out[77]:
0.5
In [78]:
f(0)
Error: Unsupported operation.  Returned None
In [79]:
f('foo')
Error: Unsupported operation.  Returned None

If we feel extra lazy we can catch all error types as follows

In [80]:
def f(x):
    try:
        return 1.0 / x
    except:
        print('Error.  Returned None')
    return None

In general it’s better to be specific

Decorators and Descriptors

Let’s look at some special syntax elements that are routinely used by Python developers

You might not need the following concepts immediately, but you will see them in other people’s code

Hence you need to understand them at some stage of your Python education

Decorators

Decorators are a bit of syntactic sugar that, while easily avoided, have turned out to be popular

It’s very easy to say what decorators do

On the other hand it takes a bit of effort to explain why you might use them

An Example

Suppose we are working on a program that looks something like this

In [81]:
import numpy as np

def f(x):
    return np.log(np.log(x))

def g(x):
    return np.sqrt(42 * x)

# Program continues with various calculations using f and g

Now suppose there’s a problem: occasionally negative numbers get fed to f and g in the calculations that follow

If you try it, you’ll see that when these functions are called with negative numbers they return a NumPy object called nan

This stands for “not a number” (and indicates that you are trying to evaluate a mathematical function at a point where it is not defined)

Perhaps this isn’t what we want, because it causes other problems that are hard to pick up later on

Suppose that instead we want the program to terminate whenever this happens, with a sensible error message

This change is easy enough to implement

In [82]:
import numpy as np

def f(x):
    assert x >= 0, "Argument must be nonnegative"
    return np.log(np.log(x))

def g(x):
    assert x >= 0, "Argument must be nonnegative"
    return np.sqrt(42 * x)

# Program continues with various calculations using f and g

Notice however that there is some repetition here, in the form of two identical lines of code

Repetition makes our code longer and harder to maintain, and hence is something we try hard to avoid

Here it’s not a big deal, but imagine now that instead of just f and g, we have 20 such functions that we need to modify in exactly the same way

This means we need to repeat the test logic (i.e., the assert line testing nonnegativity) 20 times

The situation is still worse if the test logic is longer and more complicated

In this kind of scenario the following approach would be neater

In [83]:
import numpy as np

def check_nonneg(func):
    def safe_function(x):
        assert x >= 0, "Argument must be nonnegative"
        return func(x)
    return safe_function

def f(x):
    return np.log(np.log(x))

def g(x):
    return np.sqrt(42 * x)

f = check_nonneg(f)
g = check_nonneg(g)
# Program continues with various calculations using f and g

This looks complicated so let’s work through it slowly

To unravel the logic, consider what happens when we say f = check_nonneg(f)

This calls the function check_nonneg with parameter func set equal to f

Now check_nonneg creates a new function called safe_function that verifies x as nonnegative and then calls func on it (which is the same as f)

Finally, the global name f is then set equal to safe_function

Now the behavior of f is as we desire, and the same is true of g

At the same time, the test logic is written only once

Enter Decorators

The last version of our code is still not ideal

For example, if someone is reading our code and wants to know how f works, they will be looking for the function definition, which is

In [84]:
def f(x):
    return np.log(np.log(x))

They may well miss the line f = check_nonneg(f)

For this and other reasons, decorators were introduced to Python

With decorators, we can replace the lines

In [85]:
def f(x):
    return np.log(np.log(x))

def g(x):
    return np.sqrt(42 * x)

f = check_nonneg(f)
g = check_nonneg(g)

with

In [86]:
@check_nonneg
def f(x):
    return np.log(np.log(x))

@check_nonneg
def g(x):
    return np.sqrt(42 * x)

These two pieces of code do exactly the same thing

If they do the same thing, do we really need decorator syntax?

Well, notice that the decorators sit right on top of the function definitions

Hence anyone looking at the definition of the function will see them and be aware that the function is modified

In the opinion of many people, this makes the decorator syntax a significant improvement to the language

Descriptors

Descriptors solve a common problem regarding management of variables

To understand the issue, consider a Car class, that simulates a car

Suppose that this class defines the variables miles and kms, which give the distance traveled in miles and kilometers respectively

A highly simplified version of the class might look as follows

In [87]:
class Car:

    def __init__(self, miles=1000):
        self.miles = miles
        self.kms = miles * 1.61

    # Some other functionality, details omitted

One potential problem we might have here is that a user alters one of these variables but not the other

In [88]:
car = Car()
car.miles
Out[88]:
1000
In [89]:
car.kms
Out[89]:
1610.0
In [90]:
car.miles = 6000
car.kms
Out[90]:
1610.0

In the last two lines we see that miles and kms are out of sync

What we really want is some mechanism whereby each time a user sets one of these variables, the other is automatically updated

A Solution

In Python, this issue is solved using descriptors

A descriptor is just a Python object that implements certain methods

These methods are triggered when the object is accessed through dotted attribute notation

The best way to understand this is to see it in action

Consider this alternative version of the Car class

In [91]:
class Car:

    def __init__(self, miles=1000):
        self._miles = miles
        self._kms = miles * 1.61

    def set_miles(self, value):
        self._miles = value
        self._kms = value * 1.61

    def set_kms(self, value):
        self._kms = value
        self._miles = value / 1.61

    def get_miles(self):
        return self._miles

    def get_kms(self):
        return self._kms

    miles = property(get_miles, set_miles)
    kms = property(get_kms, set_kms)

First let’s check that we get the desired behavior

In [92]:
car = Car()
car.miles
Out[92]:
1000
In [93]:
car.miles = 6000
car.kms
Out[93]:
9660.0

Yep, that’s what we want — car.kms is automatically updated

How it Works

The names _miles and _kms are arbitrary names we are using to store the values of the variables

The objects miles and kms are properties, a common kind of descriptor

The methods get_miles, set_miles, get_kms and set_kms define what happens when you get (i.e. access) or set (bind) these variables

  • So-called “getter” and “setter” methods

The builtin Python function property takes getter and setter methods and creates a property

For example, after car is created as an instance of Car, the object car.miles is a property

Being a property, when we set its value via car.miles = 6000 its setter method is triggered — in this case set_miles

Decorators and Properties

These days its very common to see the property function used via a decorator

Here’s another version of our Car class that works as before but now uses decorators to set up the properties

In [94]:
class Car:

    def __init__(self, miles=1000):
        self._miles = miles
        self._kms = miles * 1.61

    @property
    def miles(self):
        return self._miles

    @property
    def kms(self):
        return self._kms

    @miles.setter
    def miles(self, value):
        self._miles = value
        self._kms = value * 1.61

    @kms.setter
    def kms(self, value):
        self._kms = value
        self._miles = value / 1.61

We won’t go through all the details here

For further information you can refer to the descriptor documentation

Generators

A generator is a kind of iterator (i.e., it works with a next function)

We will study two ways to build generators: generator expressions and generator functions

Generator Expressions

The easiest way to build generators is using generator expressions

Just like a list comprehension, but with round brackets

Here is the list comprehension:

In [95]:
singular = ('dog', 'cat', 'bird')
type(singular)
Out[95]:
tuple
In [96]:
plural = [string + 's' for string in singular]
plural
Out[96]:
['dogs', 'cats', 'birds']
In [97]:
type(plural)
Out[97]:
list

And here is the generator expression

In [98]:
singular = ('dog', 'cat', 'bird')
plural = (string + 's' for string in singular)
type(plural)
Out[98]:
generator
In [99]:
next(plural)
Out[99]:
'dogs'
In [100]:
next(plural)
Out[100]:
'cats'
In [101]:
next(plural)
Out[101]:
'birds'

Since sum() can be called on iterators, we can do this

In [102]:
sum((x * x for x in range(10)))
Out[102]:
285

The function sum() calls next() to get the items, adds successive terms

In fact, we can omit the outer brackets in this case

In [103]:
sum(x * x for x in range(10))
Out[103]:
285

Generator Functions

The most flexible way to create generator objects is to use generator functions

Let’s look at some examples

Example 1

Here’s a very simple example of a generator function

In [104]:
def f():
    yield 'start'
    yield 'middle'
    yield 'end'

It looks like a function, but uses a keyword yield that we haven’t met before

Let’s see how it works after running this code

In [105]:
type(f)
Out[105]:
function
In [106]:
gen = f()
gen
Out[106]:
<generator object f at 0x7f848832b4f8>
In [107]:
next(gen)
Out[107]:
'start'
In [108]:
next(gen)
Out[108]:
'middle'
In [109]:
next(gen)
Out[109]:
'end'
In [110]:
next(gen)
---------------------------------------------------------------------------
StopIteration                             Traceback (most recent call last)
<ipython-input-110-6e72e47198db> in <module>()
----> 1 next(gen)

StopIteration: 

The generator function f() is used to create generator objects (in this case gen)

Generators are iterators, because they support a next method

The first call to next(gen)

  • Executes code in the body of f() until it meets a yield statement
  • Returns that value to the caller of next(gen)

The second call to next(gen) starts executing from the next line

In [111]:
def f():
    yield 'start'
    yield 'middle'  # This line!
    yield 'end'

and continues until the next yield statement

At that point it returns the value following yield to the caller of next(gen), and so on

When the code block ends, the generator throws a StopIteration error

Example 2

Our next example receives an argument x from the caller

In [112]:
def g(x):
    while x < 100:
        yield x
        x = x * x

Let’s see how it works

In [113]:
g
Out[113]:
<function __main__.g(x)>
In [114]:
gen = g(2)
type(gen)
Out[114]:
generator
In [115]:
next(gen)
Out[115]:
2
In [116]:
next(gen)
Out[116]:
4
In [117]:
next(gen)
Out[117]:
16
In [118]:
next(gen)
---------------------------------------------------------------------------
StopIteration                             Traceback (most recent call last)
<ipython-input-118-6e72e47198db> in <module>()
----> 1 next(gen)

StopIteration: 

The call gen = g(2) binds gen to a generator

Inside the generator, the name x is bound to 2

When we call next(gen)

  • The body of g() executes until the line yield x, and the value of x is returned

Note that value of x is retained inside the generator

When we call next(gen) again, execution continues from where it left off

In [119]:
def g(x):
    while x < 100:
        yield x
        x = x * x  # execution continues from here

When x < 100 fails, the generator throws a StopIteration error

Incidentally, the loop inside the generator can be infinite

In [120]:
def g(x):
    while 1:
        yield x
        x = x * x

Advantages of Iterators

What’s the advantage of using an iterator here?

Suppose we want to sample a binomial(n,0.5)

One way to do it is as follows

In [121]:
import random
n = 10000000
draws = [random.uniform(0, 1) < 0.5 for i in range(n)]
sum(draws)
Out[121]:
5000629

But we are creating two huge lists here, range(n) and draws

This uses lots of memory and is very slow

If we make n even bigger then this happens

In [122]:
n = 100000000
draws = [random.uniform(0, 1) < 0.5 for i in range(n)]

We can avoid these problems using iterators

Here is the generator function

In [123]:
def f(n):
    i = 1
    while i <= n:
        yield random.uniform(0, 1) < 0.5
        i += 1

Now let’s do the sum

In [124]:
n = 10000000
draws = f(n)
draws
Out[124]:
<generator object f at 0x7f8488069408>
In [125]:
sum(draws)
Out[125]:
5000090

In summary, iterables

  • avoid the need to create big lists/tuples, and
  • provide a uniform interface to iteration that can be used transparently in for loops

Recursive Function Calls

This is not something that you will use every day, but it is still useful — you should learn it at some stage

Basically, a recursive function is a function that calls itself

For example, consider the problem of computing $ x_t $ for some t when

$$ x_{t+1} = 2 x_t, \quad x_0 = 1 \tag{1} $$

Obviously the answer is $ 2^t $

We can compute this easily enough with a loop

In [126]:
def x_loop(t):
    x = 1
    for i in range(t):
        x = 2 * x
    return x

We can also use a recursive solution, as follows

In [127]:
def x(t):
    if t == 0:
        return 1
    else:
        return 2 * x(t-1)

What happens here is that each successive call uses it’s own frame in the stack

  • a frame is where the local variables of a given function call are held
  • stack is memory used to process function calls
    • a First In Last Out (FILO) queue

This example is somewhat contrived, since the first (iterative) solution would usually be preferred to the recursive solution

We’ll meet less contrived applications of recursion later on

Exercises

Exercise 1

The Fibonacci numbers are defined by

$$ x_{t+1} = x_t + x_{t-1}, \quad x_0 = 0, \; x_1 = 1 \tag{2} $$

The first few numbers in the sequence are $ 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55 $

Write a function to recursively compute the $ t $-th Fibonacci number for any $ t $

Exercise 2

Complete the following code, and test it using this csv file, which we assume that you’ve put in your current working directory

def column_iterator(target_file, column_number):
    """A generator function for CSV files.
    When called with a file name target_file (string) and column number
    column_number (integer), the generator function returns a generator
    that steps through the elements of column column_number in file
    target_file.
    """
    # put your code here

dates = column_iterator('test_table.csv', 1)

for date in dates:
    print(date)

Exercise 3

Suppose we have a text file numbers.txt containing the following lines

prices
3
8

7
21

Using tryexcept, write a program to read in the contents of the file and sum the numbers, ignoring lines without numbers

Solutions

Exercise 1

Here’s the standard solution

In [128]:
def x(t):
    if t == 0:
        return 0
    if t == 1:
        return 1
    else:
        return x(t-1) + x(t-2)

Let’s test it

In [129]:
print([x(i) for i in range(10)])
[0, 1, 1, 2, 3, 5, 8, 13, 21, 34]

Exercise 2

One solution is as follows

In [130]:
def column_iterator(target_file, column_number):
    """A generator function for CSV files.
    When called with a file name target_file (string) and column number
    column_number (integer), the generator function returns a generator
    which steps through the elements of column column_number in file
    target_file.
    """
    f = open(target_file, 'r')
    for line in f:
        yield line.split(',')[column_number - 1]
    f.close()

dates = column_iterator('test_table.csv', 1)

i = 1
for date in dates:
    print(date)
    if i == 10:
        break
    i += 1
Date
2009-05-21
2009-05-20
2009-05-19
2009-05-18
2009-05-15
2009-05-14
2009-05-13
2009-05-12
2009-05-11

Exercise 3

Let’s save the data first

In [131]:
%%file numbers.txt
prices
3
8

7
21
Writing numbers.txt
In [132]:
f = open('numbers.txt')

total = 0.0
for line in f:
    try:
        total += float(line)
    except ValueError:
        pass

f.close()

print(total)
39.0