A safe max() function for empty lists

python list exception error-handling max

Alexander McFarlane · Mar 22, 2016 · Viewed 28.5k times · Source

Evaluating,

max_val = max(a)

will cause the error,

ValueError: max() arg is an empty sequence

Is there a better way of safeguarding against this error other than a try, except catch?

a = []
try:
    max_val = max(a)
except ValueError:
    max_val = default

Answer

In Python 3.4+, you can use default keyword argument:

>>> max([], default=99)
99

In lower version, you can use or:

>>> max([] or [99])
99

NOTE: The second approach does not work for all iterables. especially for iterator that yield nothing but considered truth value.

>>> max(iter([]) or 0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: max() arg is an empty sequence