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Modules and namespaces

Modules

Most of the functionality in Python is provided by modules. The Python Standard Library is a large collection of modules that provides cross-platform implementations of common facilities such as access to the operating system, file I/O, string management, network communication, math, web-scraping, text manipulation, machine learning and much more.

To use a module in a Python program it first has to be imported. A module can be imported using the import statement. For example, to import the module math, which contains many standard mathematical functions, we can do:

import math

x = math.cos(2 * math.pi)

print(x)

1.0

This includes the whole module and makes it available for use later in the program. Alternatively, we can chose to import all symbols (functions and variables) in a module so that we don’t need to use the prefix “math.” every time we use something from the math module:

from math import *

x = cos(2 * pi)

print(x)

1.0

This pattern can be very convenient, but in large programs that include many modules it is often a good idea to keep the symbols from each module in their own namespaces, by using the import math pattern. This would eliminate potentially confusing problems.

Namespaces

A namespace is an identifier used to organize objects, e.g. the methods and variables of a module. The prefix math. we have used in the previous section is such a namespace. You may also create your own namespace for a module.

import math as m

x = m.sqrt(2)

print(x)

1.4142135623730951

You may also only import specific functions of a module.

from math import sinh as mysinh
from math import pi as mypi

mysinh(mypi)

11.548739357257746

Contents of a module

Once a module is imported, we can list the symbols it provides using the dir function:

import math

print(dir(math))

['__doc__', '__file__', '__loader__', '__name__', '__package__', '__spec__', 'acos', 'acosh', 'asin', 'asinh', 'atan', 'atan2', 'atanh', 'ceil', 'copysign', 'cos', 'cosh', 'degrees', 'e', 'erf', 'erfc', 'exp', 'expm1', 'fabs', 'factorial', 'floor', 'fmod', 'frexp', 'fsum', 'gamma', 'gcd', 'hypot', 'inf', 'isclose', 'isfinite', 'isinf', 'isnan', 'ldexp', 'lgamma', 'log', 'log10', 'log1p', 'log2', 'modf', 'nan', 'pi', 'pow', 'radians', 'remainder', 'sin', 'sinh', 'sqrt', 'tan', 'tanh', 'tau', 'trunc']

And using the function help we can get a description of each function (almost .. not all functions have docstrings, as they are technically called, but the vast majority of functions are documented this way).

help(math.log)

Help on built-in function log in module math:

log(...)
    log(x, [base=math.e])
    Return the logarithm of x to the given base.

    If the base not specified, returns the natural logarithm (base e) of x.

math.log(10)

2.302585092994046

math.log(8, 2)

3.0

We can also use the help function directly on modules: Try

help(math)

Some very useful modules form the Python standard library are os, sys, math, shutil, re, subprocess, multiprocessing, threading.

A complete lists of standard modules for Python 3 is available at http://docs.python.org/3/library/ .