Import-ant Decisions by Allison Kaptur

Suppose `import` didn't exist, and we had to invent it from scratch. We'll look at the problem - code sharing and reuse across modules - and different ways we could solve it. We'll come up with `import` from parallel universes and reinvent python's actual implementation.

Transcript

1. Import-ant Decisions
   Allison Kaptur!
   PyCon 2014!
   allison.kaptur@gmail.com!
   github.com/akaptur!
   @akaptur
   

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2. THIS IS A
   COUP
   

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3. import
   

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4. The problem
   

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5. Copy & paste
   

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6. A magical copy & paste
   

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7. A magical copy & paste
   Implemented outside of Python
   (e.g. a bash script):
   !
   1. Take your .py file
   2. Look for “%magical_paste useful”
   3. Find the file useful.py
   4. Replace the magical_paste line with the contents of
   useful.py
   

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8. We have problems
   1. Static
   2. Name collisions
   3. Executes multiple times
   4. Rigid
   

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9. We have problems
   1. Static
   2. Name collisions
   3. Executes multiple times
   4. Rigid
   

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10. Call a function
    

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11. exec
    

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12. Call a function
    

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13. We have problems
    1. Static: use Python at run-time
    2. Name collisions
    3. Executes multiple times
    4. Rigid
    

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14. We have problems
    1. Static: use Python at run-time
    2. Name collisions
    3. Executes multiple times
    4. Rigid
    

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16. Solution: namespaces
    Montréal Montréal
    Images:
    http://en.wikipedia.org/wiki/Montr%C3%A9al,_Ard%C3%A8che
    http://en.wikipedia.org/wiki/Montreal
    

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17. Solution: namespaces
    Montréal, France Montréal, Canada
    

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18. Solution: namespaces
    France.Montréal Canada.Montréal
    

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19. Solution: namespaces
    France Canada
    

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20. Module
    

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21. Module
    

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22. exec with a namespace
    

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23. exec with a namespace
    

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24. exec with a namespace
    

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25. Adding a namespace
    

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26. Adding a namespace
    (This is valid code)
    

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27. We have problems
    1. Static: use Python at run-time
    2. Name collisions: use namespaces and modules
    3. Executes multiple times
    4. Rigid
    

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28. We have problems
    1. Static: use Python at run-time
    2. Name collisions: use namespaces and modules
    3. Executes multiple times
    4. Rigid
    

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29. Executes multiple times
    

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30. Memoize
    

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31. Memoize
    

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32. We have problems
    1. Static: use Python at run-time
    2. Name collisions: use namespaces and modules
    3. Executes multiple times: memoize
    4. Rigid
    

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33. We have problems
    1. Static: use Python at run-time
    2. Name collisions: use namespaces and modules
    3. Executes multiple times: memoize
    4. Rigid
    

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34. Options?
    

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35. We have problems
    1. Static: use Python at run-time
    2. Name collisions: use namespaces and modules
    3. Executes multiple times: memoize
    4. Rigid Ugly
    

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36. Keywords:
    syntactic sugar
    

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37. We have problems
    1. Static: use Python at run-time
    2. Name collisions: use namespaces and modules
    3. Executes multiple times: memoize
    4. Rigid: use keywords for flexible syntax
    

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38. check sys.modules to see if
    name is already imported
    make an empty module
    (a namespace)
    find the source code
    execute the source code in
    the new, empty module
    bind name in the caller’s
    namespace
    Actual
    import
    insert the module into
    sys.modules
    

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39. (Some) things we haven’t done
    1. Created real modules
    2. Created packages
    3. Worried about loaders
    4. Worried about error handling
    

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40. Other solutions
    

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41. “magical copy & paste”
    #include in C
    

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42. Ruby modules
    

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43. Modula
    “A module is a set of procedures, data types, and
    variables, where the programmer has precise control
    over the names that are imported from and exported to
    the environment.”
    !
    - N. Wirth, “Modula: A Language for Modular
    Multiprogramming” (1976)
    

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44. There are these two young fish swimming along and
    they happen to meet an older fish swimming the other
    way, who nods at them and says "Morning, boys.
    How's the water?" And the two young fish swim on for
    a bit, and then eventually one of them looks over at
    the other and goes "What the hell is water?”
    !
    - David Foster Wallace, 2005
    

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45. Invisible blind spots of
    Python programmers when
    it comes to ‘import’
    Allison Kaptur!
    PyCon 2014!
    github.com/akaptur!
    @akaptur
    

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46. Questions
    Allison Kaptur!
    PyCon 2014!
    github.com/akaptur!
    @akaptur
    

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