Idiomatic Python

2020-11-03 394 words 2 minutes

Contents

EAFP > LBYL

Just try to do what you expect to work and handle exceptions later if they occurr - “it’s easier to ask for forgiveness than permission” (EAFP) rather than “look before you leap” (LBYL).

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def dont(m, n):
    if n == 0:
        print("Can't divide by 0")
        return None
    return m / n


def do(m, n):
    try:
        return m / n
    except ZeroDivisionError:
        print("Can't divide by 0")
        return None

Other examples

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try:
    dangerous_code()
except SomeError:
    handle_error()
except (OneError, OtherError):
    handle_multiple_errors()

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try:
    os.remove("filename")
except FileNotFoundError:
    pass

Long strings

Use a separate set of parenthesis for each line and wrap them all in a set of parenthesis.

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my_very_big_string = (
    "For a long time I used to go to bed early. Sometimes, "
    "when I had put out my candle, my eyes would close so quickly "
    "that I had not even time to say “I’m going to sleep.”"
)

Generator expression instead of if-else membership testing

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cities = ["Berlin", "New York", "London"]
text = "Time Square is in New York"


def dont(cities, text):
    for city in cities:
        if city in text:
            return True


def do(cities, text):
    return any(city in text for city in cities)

Negated logical operators

Use if x not in y instead of if not x in y, since they are semantically identical but the former makes it clear that not in is a single operator and reads like English. (Idion from PEP8, rationale from Alex Martelli)

Using argparse

There are different ways to use argparse. I usually use

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def parse_args(argv):
    parser = argparse.ArgumentParser()
    parser.add_argument("path")


def main(path):
    pass
    # something using path

I use this as my default approach because it’s appropriately simple for my normal use case and it makes testing main() easy. Alternatively, I sometimes use the below (based on this discussion)

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def main(argv=None):
    if argv is None:
        argv = sys.argv
    args = parse_args(argv)
    # do stuff with args

Use `ast.literal_eval()` instead of `eval()`

Why? Basically, because eval is very dangerous and would happile evaluate a string like os.system(rm -rf /), while ast.literal_eval will only evaluate Python literals.