Diving deep on how imports work in Python

Talk was given on PyCon Taiwan 2017

Talk link: https://tw.pycon.org/2017/en-us/events/talk/342865744498786414/

Transcript

1. Diving deep on how imports
   work in Python
   PyCon Taiwan, 2017
   presented by Tasdik Rahman (@tasdikrahman)
   

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2. But Why?
   

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3. Modules
   

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4. Terminology
   •Loader
   Loads a module
   •Finder
   Finds a module
   

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5. Module search path
   

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6. sys.path
   

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7. >>> import sys
   >>>
   >>> pprint(sys.path)
   ['',
   '/usr/local/Cellar/python3/3.6.0/Frameworks/
   Python.framework/Versions/3.6/lib/python36.zip',
   '/usr/local/Cellar/python3/3.6.0/Frameworks/
   Python.framework/Versions/3.6/lib/python3.6',
   '/usr/local/Cellar/python3/3.6.0/Frameworks/
   Python.framework/Versions/3.6/lib/python3.6/lib-
   dynload',
   '/usr/local/lib/python3.6/site-packages']
   >>>
   

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8. Compiled Python files
   

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9. • __pycache__ under a name like module.version.pyc
   • __pycache__/spam.cpython-33.pyc
   • platform independent
   • regular lookup with source checking for modification.
   

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10. import foo
    

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11. 2 step process
    • find a module, loading and
    initialising it if necessary
    • define a name or names in the local
    namespace for the scope where the
    "import" statement occurs.
    

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12. If module is retrieved
    successfully
    

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13. import foo # foo imported and bound locally
    import foo.bar.baz 

    # foo.bar.baz imported, foo bound locally
    import foo.bar.baz as fbb 

    # foo.bar.baz imported and bound as fbb
    

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14. from foo.bar import baz
    

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15. • find the module specified in the "from" clause, loading and
    initialising if necessary
    • for each of the identifiers specified in the "import" clauses:
    1. check if the imported module has an attribute by that name
    2. attempt to import a submodule with that name and check the
    imported module again for that attribute
    3. if the attribute is not found, "ImportError" is raised.
    

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16. from foo.bar import baz
    # foo.bar.baz imported and bound as baz
    from foo import attr
    # foo imported and foo.attr bound as attr
    

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17. from foo import *
    

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18. Packages
    

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19. Advantages?
    

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20. car/ Top-level package
    __init__.py Initialize the car package
    engine/ Subpackage for engine behaviour
    __init__.py
    rev.py
    temperature.py
    fuel.py
    coolant.py
    ...
    transmission/ Subpackage for transmission
    __init__.py
    forward.py
    reverse.py
    ...
    infotainment/ Subpackage for infotainment system
    __init__.py
    music.py
    reverseparking.py
    chilledbeer.py
    ...
    

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21. __init__.py
    

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22. import car.engine.rev
    car.engine.rev.spin(…)
    # OR
    from car.engine import rev
    rev.spin(...)
    # OR
    from car.engine.rev import spin
    spin(...)
    

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23. __all__
    

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24. from car.engine import *
    # car/engine/__init__.py
    __all__ = ["temperature", "fuel", "coolant"]
    

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25. What if there is no
    __all__ ?
    

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26. Intra-package
    reference
    

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27. # car/engine/temperature
    from car.transmission import forward
    from . import forward
    from .. import transmission
    from ..infotainment import music
    

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28. Some takeaways
    

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29. Questions?

    

    Would be happy to take them :)
    http://tasdikrahman.me/

    @tasdikrahman
    

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