How to Write Functions in Python

PyCon JP Reject Conference 2017

Transcript

1. How to Write Functions in Python
   Hayao Suzuki
   PyCon JP Reject Conference 2017 at TECH PLAY SHIBUYA
   September 6, 2017
   

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2. Who Am I?
   About Me
   Name Hayao Suzuki (@CardinalXaro)
   Blog http://xaro.hatenablog.jp/
   Major Mathematics (Combinatorics, Number Theory)
   Work Python Programmer at iRidge, Inc.
   Reviewed Books
   Effective Python (O’Reilly Japan)
   ΞϧΰϦζϜΫΠοΫϦϑΝϨϯε ୈ 2 ൛ (O’Reilly Japan)
   ॳΊͯͷ PHP (O’Reilly Japan)
   Effective Debugging (O’Reilly Japan)
   ͢Β͢ΒΘ͔Δ Python ʢᠳӭࣾʣ
   Python ͱ JavaScript Ͱ͸͡ΊΔσʔλϏδϡΞϥΠθʔγϣϯ
   (O’Reilly Japan)
   2 / 15
   

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3. Today’s Theme
   How to Write Functions in Python
   Surface-Level Improvements
   The Power of Keyword Arguments
   Stateful Functions
   3 / 15
   

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4. Surface-Level Improvements
   I’ve bought this book twice.
   4 / 15
   

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5. Surface-Level Improvements
   Ugly Example
   def f(ns):
   sum=0
   for i in ns:
   sum=sum+i
   return sum
   print(f([1,2,3,4,5]))
   5 / 15
   

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6. Surface-Level Improvements
   Improvement Point
   Follow the PEP 8.
   Follow the PEP 8 Example
   def f(ns):
   sum = 0
   for i in ns:
   sum = sum + i
   return sum
   print(f([1, 2, 3, 4, 5]))
   6 / 15
   

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7. Surface-Level Improvements
   Improvement Point
   Consider namespace (e.g. sum is a builtin-function).
   sum ! total
   def f(ns):
   total = 0
   for i in ns:
   total = total + i
   return total
   print(f([1, 2, 3, 4, 5]))
   7 / 15
   

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8. Surface-Level Improvements
   Improvement Point
   Packing Iinformation into names.
   Improve names
   def summation(numbers):
   total = 0
   for number in numbers:
   total = total + number
   return total
   print(summation([1, 2, 3, 4, 5]))
   8 / 15
   

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9. Surface-Level Improvements
   Improvement Point
   Docstring, Type hints.
   Readble Example
   from typing import List
   def summation(numbers: List) -> int:
   """Calculate summation of numbers"""
   total = 0
   for number in numbers:
   total = total + number
   return total
   print(summation([1, 2, 3, 4, 5]))
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10. The Power of Keyword Arguments
    Keyword Args with Default Value
    Add new behaivors to a function.
    Default Value
    from typing import List
    def summation(numbers: List, modulo: int = 1) -> int:
    """Calculate summation of numbers"""
    total = 0
    for number in numbers:
    total = total + number
    return total % modulo
    print(summation([1, 2, 3, 4, 5])) # No Change!
    print(summation([1, 2, 3, 4, 5], modulo=4)) # 15 % 5 = 3
    10 / 15
    

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11. The Power of Keyword Arguments
    Keyword Only Args (Python 3 Only)
    Force callers to supply keyword args.
    Keyword Only Args
    from typing import List
    def summation(numbers: List, *, modulo: int = 1) -> int:
    """Calculate summation of numbers"""
    total = 0
    for number in numbers:
    total = total + number
    return total % modulo
    print(summation([1, 2, 3, 4, 5], modulo=5)) # 0
    print(summation([1, 2, 3, 4, 5], 5)) # raise TypeError
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12. The Power of Keyword Arguments
    Mutable Default Value
    Default value is only executed a single time when the function is
    defined.
    The Same Timestamp
    >>> from datetime import datetime
    >>> from time import sleep
    >>> def log(message, when=datetime.now()):
    ... print(f"{message}: {when}")
    ...
    >>> log("Good Morning!")
    Good Morning!: 2017-09-03 22:20:40.786377
    >>> sleep(1.0)
    >>> log("͓͸Α͏͍͟͝·͢")
    ͓͸Α͏͍͟͝·͢: 2017-09-03 22:20:40.786377
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13. The Power of Keyword Arguments
    Mutable Default Value
    Use None to Mutable Default Value.
    The Correct Timestamp
    >>> from datetime import datetime
    >>> from time import sleep
    >>> def log(message, when=None):
    ... when = datetime.now() if when is None else when
    ... print(f"{message}: {when}")
    ...
    >>> log("Good Morning!")
    Good Morning!: 2017-09-03 22:20:41.786424
    >>> sleep(1.0)
    >>> log("͓͸Α͏͍͟͝·͢")
    ͓͸Α͏͍͟͝·͢: 2017-09-03 22:20:42.787629
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14. Stateful Functions
    Stateful Function
    Implement callable class.
    Implements Summation with Histories
    class Summation:
    def __init__(self):
    self.histories = list()
    def __call__(self, numbers, *, modulo=1):
    total = sum(numbers) % modulo
    self.histories.append((numbers, modulo, total))
    return total
    summation = Summation()
    print(summation([1, 2, 3, 4, 5]))
    print(summation([1, 2, 3], modulo=3))
    print(summation.histories)
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15. Summary
    Summary
    If you don’t have "The Art of Readable Code", go to a
    bookstore and buy it.
    Follow the PEP 8. Consider built-in namespaces.
    Keyword Arguments are very powerful. Be careful of mutable
    default values.
    If you want to implement stateful functions, we recommend
    callable class instead of closures.
    References
    Ϧʔμϒϧίʔυ (O’Reilly Japan)
    Effective Python (Addison-Wesley)
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