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# How to Write Functions in Python

PyCon JP Reject Conference 2017

#### HayaoSuzuki

September 06, 2017

## Transcript

1. ### How to Write Functions in Python Hayao Suzuki PyCon JP

Reject Conference 2017 at TECH PLAY SHIBUYA September 6, 2017
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
3. ### Today’s Theme How to Write Functions in Python Surface-Level Improvements

The Power of Keyword Arguments Stateful Functions 3 / 15

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
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
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
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
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])) 9 / 15
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
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 11 / 15
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 12 / 15
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 13 / 15
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) 14 / 15
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) 15 / 15