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How to Write Functions in Python
Hayao Suzuki
PyCon JP Reject Conference 2017 at TECH PLAY SHIBUYA
September 6, 2017
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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 Ͱ͡ΊΔσʔλϏδϡΞϥΠθʔγϣϯ
(O’Reilly Japan)
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Today’s Theme
How to Write Functions in Python
Surface-Level Improvements
The Power of Keyword Arguments
Stateful Functions
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Surface-Level Improvements
I’ve bought this book twice.
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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]))
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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]))
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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]))
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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]))
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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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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
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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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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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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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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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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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