PYL-C1802 · Built-in function `len` used as condition

pnijhara / gitsome

Active

Built-in function len used as condition PYL-C1802

Performance

Major

4 years ago — 4 years old

Do not use len(SEQUENCE) without comparison to determine if a sequence is empty

892    with p.group(len(name) + 1, name + "(", ")"):
893        if cycle:
894            p.text("...")
895        elif len(obj):896            p.pretty(dict(obj))

Do not use len(SEQUENCE) without comparison to determine if a sequence is empty

xonsh/pretty.py

874    with p.group(len(name) + 1, name + "(", ")"):
875        if cycle:
876            p.text("...")
877        elif len(obj):878            p.pretty(list(obj.items()))
879
880

Do not use len(SEQUENCE) without comparison to determine if a sequence is empty

xonsh/environ.py

1552        with p.group(0, name + "(", ")"):
1553            if cycle:
1554                p.text("...")
1555            elif len(self):1556                p.break_()
1557                p.pretty(dict(self))
1558

Do not use len(SEQUENCE) without comparison to determine if a sequence is empty

xonsh/environ.py

 354        with p.group(0, name + "(", ")"):
 355            if cycle:
 356                p.text("...")
 357            elif len(self): 358                p.break_()
 359                p.pretty(dict(self))
 360

Do not use len(SEQUENCE) without comparison to determine if a sequence is empty

xonsh/aliases.py

162        with p.group(0, name + "(", ")"):
163            if cycle:
164                p.text("...")
165            elif len(self):166                p.break_()
167                p.pretty(dict(self))
168

Description

Using the len function to check if a sequence is empty is not idiomatic and can be less performant than checking the truthiness of the object.

len doesn't know the context in which it is called, so if computing the length means traversing the entire sequence, it must; it doesn't know that the result is just being compared to 0. Computing the boolean value can stop after it sees the first element, regardless of how long the sequence actually is.

Not preferred:

if not len(my_sequence):
    print("Empty sequence.")
else:
    print("Sequence is not empty.")

Preferred:

if not my_sequence:
    print("Empty sequence.")
else:
    print("Sequence is not empty.")