空列表的安全 max()函数

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

The max of an empty sequence "should" be an infinitely small thing of whatever type the elements of the sequence have. Unfortunately, (1) with an empty sequence you can't tell what type the elements were meant to have and (2) there is, e.g., no such thing as the most-negative integer in Python.

So you need to help max out if you want it to do something sensible in this case. In recent versions of Python there is a default argument to max (which seems to me a misleading name, but never mind) which will be used if you pass in an empty sequence. In older versions you will just have to make sure the sequence you pass in isn't empty -- e.g., by oring it with a singleton sequence containing the value you'd like to use in that case.

[EDITED long after posting because Yaakov Belch kindly pointed out in comments that I'd written "infinitely large" where I should have written "infinitely small".]

In versions of Python older than 3.4 you can use itertools.chain() to add another value to the possibly empty sequence. This will handle any empty iterable but note that it is not precisely the same as supplying the default argument as the extra value is always included:

>>> from itertools import chain
>>> max(chain([42], []))
42

But in Python 3.4, the default is ignored if the sequence isn't empty:

>>> max([3], default=42)
3

Considering all the comments above it can be a wrapper like this:

def max_safe(*args, **kwargs):
"""
Returns max element of an iterable.


Adds a `default` keyword for any version of python that do not support it
"""
if sys.version_info < (3, 4):  # `default` supported since 3.4
if len(args) == 1:
arg = args[0]
if 'default' in kwargs:
default = kwargs.pop('default')
if not arg:
return default


# https://stackoverflow.com/questions/36157995#comment59954203_36158079
arg = list(arg)
if not arg:
return default


# if the `arg` was an iterator, it's exhausted already
# so use a new list instead
return max(arg, **kwargs)


return max(*args, **kwargs)

Another solution could be by using ternary operators:

nums = []
max_val = max(nums) if nums else 0

or

max val = max(iter(nums) if nums else [0])

_DEFAULT = object()


def max_default(*args, **kwargs):
"""
Adds support for "default" keyword argument when iterable is empty.
Works for any iterable, any default value, and any Python version (versions >= 3.4
support "default" parameter natively).


Default keyword used only when iterable is empty:


>>> max_default([], default=42)
42


>>> max_default([3], default=42)
3


All original functionality is preserved:


>>> max_default([])
Traceback (most recent call last):
ValueError: max() arg is an empty sequence


>>> max_default(3, 42)
42
"""


default = kwargs.pop('default', _DEFAULT)
try:
return max(*args, **kwargs)
except ValueError:
if default is _DEFAULT:
raise
return default

Bonus:

def min_default(*args, **kwargs):
"""
Adds support for "default" keyword argument when iterable is empty.
Works for any iterable, any default value, and any Python version (versions >= 3.4
support "default" parameter natively).


Default keyword used only when iterable is empty:


>>> min_default([], default=42)
42


>>> min_default([3], default=42)
3


All original functionality is preserved:


>>> min_default([])
Traceback (most recent call last):
ValueError: min() arg is an empty sequence


>>> min_default(3, 42)
3
"""


default = kwargs.pop('default', _DEFAULT)
try:
return min(*args, **kwargs)
except ValueError:
if default is _DEFAULT:
raise
return default

Can create simple lambda to do this:

get_max = lambda val_list: max([ val for val in val_list if val is not None ]) if val_list else None

You can call it this way:
get_max(your_list)