msolorio / python_imports_lesson

Learn once and for all how to use relative and absolute imports in Python 3

Python 3 Imports for 6 Year Olds

Learn once and for all how to use relative and absolute imports in Python 3

Quick Notes

• When a file is run directly, it is denoted as the __main__ file.
• Absolute imports specify a path relative to the __main__ file.
• Relative imports specify a path relative to the current file of the import statement.
• The __main__ file can only include absolute imports and not relative imports.
• Prefer absolute imports when possible, as scripts that use relative imports cannot be run directly.
• Python 3.3 and above only allows explicit relative imports, and not implicit relative imports, so those will not be covered.

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Assume this file structure.

project
├── main.py
└── my_package
    ├── my_module.py
    └── my_other_module.py

__main__ File

• When a file is run directly it is denoted as the "__main__" file. That is, __name__ is set to __main__ for that file.
• If a file is run indirectly, by being imported from another file, __name__ is set to the name of the module, For example, __my_module__.

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Absolute Import - Case #1

in main.py

from my_package.my_module import my_var

print(my_var)

in my_package/my_module.py

my_var = 'a variable'

Now run main.py

python3 main.py

In Case #1, we include the absolute path to my_module.py and import my_var

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Absolute Import - Case #2

in main.py

from my_package.my_module import my_var

print(my_var)

in my_package/my_module.py

from my_package.my_other_module import my_other_var

my_var = 'some text and ' + my_other_var

in my_package/my_other_module.py

my_other_var = 'my other variable'

Now run main.py

python3 main.py

In Case #2,

• we include an absolute path to my_module within main.py, as well as an absolute path my_other_module in my_module.py.
• The absolute import specifies the path relative to the __main__ file, the file being run directly from the command line, in this case, main.py.
• If we were to try and run my_package/my_module.py directly, the import statement would break. The absolute path is assuming main.py is the __main__ file.

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Relative Import - Case #1

A relative import specifies a path relative to the current file of the import statement. Let us now assume this code, changing the import statement in my_package/my_module.py

in main.py

from my_package.my_module import my_var

print(my_var)

in my_package/my_module.py

from .my_other_module import my_other_var

my_var = 'some text and ' + my_other_var

in my_package/my_other_module.py

my_other_var = 'my other variable'

Now run main.py

python3 main.py

In Case #1,

• The import statement in my_package/my_module.py has been changed to a relative import.
• The import statement specifies a file path relative to the current location of the import statement.
• This code should still function properly when running python3 main.py.
• If we were to try to run my_package/my_module.py directly, the code would break. my_module.py would then be the __main__ file and relative imports are not allowed in the __main__ file.

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Relative Imports - Other Uses

Stepping into a nested directories

from .dir1.dir2.my_module import my_var

Steping out of a directory - Here we step out of our current directory to access a file at a higher level

from ..main import other_var

Stepping out of 2 Directories

from ...main import other_var

• Using relative paths we can not run these files directly by calling python3 name_of_file.py.
• We can however run the file as a module using the -m flag.
  • as in python3 -m name_of_file
  • notice we do not include the file extension here.