Modularity¶

Modules¶

_docs/words.py

python3 words.py

Functions¶

Defining Functions¶

>>> def square(x):
...     return x * x
...
>>> square(5)
25
>>> def launch_missiles():
...     print("Missiles launched!")
...
>>> launch_missiles()
Missiles launched!
>>> def even_or_odd(n):
...     if n % 2 == 0:
...             print("even")
...             return
...     print("odd")
...
>>> w = even_or_odd(31)
odd
>>> w is None
True
>>> def nth_root(radicand, n):
...     return radicand ** (1/n)
...
>>> nth_root(16,2)
4.0
>>> nth_root(27,3)
3.0

dunder

Our way of pronouncing special names
A portmanteau of "double underscore"
Instead of "underscore underscore name underscore underscore" we'll say "dunder name"

name¶

__name__

Specially named variable allowing us to detect whether a module is run as a script or imported into another module.
if __name__ == '__main__': if name = main, execute function, if name != main, it knows it's being imported into another module, not executed. By using this statement, the python script can be executed as a script or module.
__name__ will be set as __main__ depending on how it's used - if running python words.py, __main__ will return as __name__ instead.

print(__name__)

The Python Execution Model¶

def is a statement
Top-level functions are defined when a module is imported or run

Module Script or Program¶

Python module - Convenient import with API
Python script - Convenient execution from the command line (scripts should be importable)
Python program - Perhaps composed of many modules

Command Line Arguments¶

import sys
from urllib.request import urlopen

def fetch_words(url):
    story = urlopen(url)
    story_words = []
    for line in story:
        line_words = line.decode('utf-8').split()
        for word in line_words:
            story_words.append(word)
    story.close()
    return story_words

def print_items(items):
    for item in items:
        print (item)

def main():
    words = fetch_words()
    print_words(words)

if __name__ == '__main__':
    main(sys.argv[1])

Docstrings¶

Literal strings which document functions, modules, and classes.
They must be the first statement in the blocks for these constructs.
PEP 257
- Official Python convention for docstrings.
- But... not widely adopted.
Sphinx
- Tool to create HTML documentation from Python docstrings.

""" Retrieve and print words from a URL.

Usage:

  python3 words.py <URL>
"""

import sys
from urllib.request import urlopen


def fetch_words(url):
    """Fetch a list of words from a URL.

      Args:
        url: The URL of a UTF-8 text-document.

      Returns:
        A list of strings containing words from the document
    """
    story = urlopen(url)
    story_words = []
    for line in story:
        line_words = line.decode('utf-8').split()
        for word in line_words:
            story_words.append(word)
    story.close()
    return story_words


def print_items(items):
    """Print items one per line.

        Args:
            An iterable series of printable items.
    """
    for item in items:
        print (item)


def main():
    """Print each word from a text document from at a URL.

        Args:
            url: The URL of a UTF-8 text document.
    """
    words = fetch_words()
    print_words(words)


if __name__ == '__main__':
    main(sys.argv[1])

>>> import words
>>> help(words)
Help on module words:
NAME
    words - Retrieve and print words from a URL.

DESCRIPTION
    Usage:

        python3 words.py <URL>

FUNCTIONS
    fetch_words(url)
        Fetch a list of words from a URL.

        Args:
            url: The URL of a UTF-8 text document.

        Returns:
            A list of strings containing the words from the document

    main(url)
        Print each word from a text document from at a URL.

Comments¶

Code is ideally clear enough without ancillary explanation
Sometimes you need to explain why your code is written as it is
Comments in Python start with # and extend to the end of the line

if _name_ =='__main__':
main(sys.argv[1]) # The Oth arg is the module filename.]

Shebang¶

Special Comment
#!/usr/bin/env python
#!/usr/bin/env python3

Pylauncher¶

On Windows

Associated with *.py files
Executable is py.exe and is on the PATH
Parse the shebang and locate Python
PEP397