Python imports, reloads, and associated dangers by Sorcha Bowler

This talk is part of "PyLadies Dublin Aug Virtual Event - Poetry | Python Imports"
- When: Tuesday, August 18, 2020
- Event Page: https://www.meetup.com/PyLadiesDublin/events/271722192/

Talk Description:
I lost a half a day's work to trying to find a bug that didn't exist because reloading imports didn't do what I thought it would. I'm here to save you that fate.

About Sorcha Bowler:
Sorcha studied computer science in college, did a non-coding job for many years, taught themself python, got a coding job, and now writes code for Google and is still rather surprised by that.

Transcript

1. Python imports, reloads
   (and associated dangers)
   

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2. A quick aside about Enums
   ● Enum is short for Enumerated Type
   ● I’ve come across them far more in other languages, but they’re in Python
   >>> from enum import Enum
   >>> class Color(Enum):
   ... RED = 1
   ... GREEN = 2
   ... BLUE = 3
   ● Now you can use Color.RED as a constant
   ● Color.RED is always Color.RED. Color.GREEN is always Color.GREEN
   

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3. A rough
   reconstruction of
   a work day I had.
   (with less swearing)
   

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4. Let’s add a few
   quick prints and
   try that again.
   

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5. A quick aside about Identity in python
   ● Python separates out the ideas of ‘is the same as’ (= =) and
   ‘is the same actual thing as’ (is)
   ○ Mostly we come across this as being told that we should use ‘is None’ instead of ‘== None’
   because something something custom comparison operators
   ● You can get the identity of an object with the id() function
   ● Interestingly, python stores a single copy of the integers from -5 to 256, but
   creates a new object for integers outside that range each time, so
   >>> a = 256
   >>> b = 256
   >>> a is b
   True
   >>> a = 257
   >>> b = 257
   >>> a is b
   False
   

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6. Once more,
   with id()
   

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7. Conclusions
   ● importlib.reload will not reload things loaded as ‘from import
   ○ One way around this (I learned, while writing these slides) would have been this:
   ■ >>> import file_b
   ■ >>> file_b.MyEnum
   ■ Instead of this
   ■ >>> from file_b import MyEnum
   ■ >>> MyEnum
   ● Enum identity is set when the file it is in is imported
   

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8. Thanks!
   saoili@
   

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