python-package-module-notes.md

Python Package and Modules Notes

Basic module usage

Code in a file is referred to as a module. You can encapsolate code in modules and then import them into other files.

Say you have a file foo.py with:

def bar():
    print('baz')

In another script in the same directory, you can use the bar function like this:

import foo

foo.bar()
#=> baz

You can also import the function like this:

from foo import bar

bar()
#=> baz

Importing from a directory (packages)

If the foo module is located in a directory, like biz

├── biz
│   └── foo.py
└── main.py

... you can import it like this

from biz.foo import bar

bar()

... or, like this

import biz.foo

biz.foo.bar()

This is referred to as a "package". In earlier versions of Python you needed to include an __init__.py file in a directory to make it a package. However this is implicit now and not required.

The __init__.py file can be a good place to put initialization code, if you're package needs any. The file is only loaded and executed the first time a package is imported.

Inside of a package you need to import things slighly differently. Lets say you have a file biz/baz.py and you need to import biz/foo.py into it to get the bar method:

# baz.py
from .foo import bar

bar()

The dot prefix tells the Python importer to look in the current package.

Search path

When a module name is used, Python will first look locally and then it will look up a series of directories, which you can access with the sys module:

import sys

print(sys.path)

Find out what a module defines

You can use the dir method to figure out what a module defines:

import biz.foo

dir(biz.foo)
#=> ['__builtins__', '__cached__', '__doc__', '__file__', '__loader__', '__name__', '__package__', '__spec__', 'bar']

Running code only if not being imported

Sometimes you only want to code to execute if it's being run directly, not when it's being imported.

From the documentation:

'main' is the name of the scope in which top-level code executes. A module’s name is set equal to 'main' when read from standard input, a script, or from an interactive prompt.

def main():
    pass

if __name__ == "__main__":
    main()