问题
I'm confused in this code, the allmax passes hand_rank function as the key, but in the definition of allmax, it sets the key to be None, then how the hand_rank passed to this allmax function?
def poker(hands):
"Return a list of winning hands: poker([hand,...]) => [hand,...]"
return allmax(hands, key=hand_rank)
def allmax(iterable, key=None):
"Return a list of all items equal to the max of the itterable."
result, maxval = [],None
key = key or (lambda x:x)
for x in iterable:
xval = key(x)
if not result or xval > maxval:
result,maxval = [x],xval
elif xval == maxval:
result.append(x)
return result
回答1:
That's a default parameter. key would only default to None, if hand_rank was never passed, or was passed empty. You can set a default parameter to be pretty much anything,
def allmax(iterable, key=[])
However in this case, it looks like you want to avoid updating an empty list. If key was set to [ ], it would add to the list on later calls, rather than start with a fresh list.
Here's a good explanation of mutable default parameters, and why None is used to stop unexpected behaviour. And here is someone else on Stack Overflow having the empty list parameter problem.
回答2:
Default value for parameter in the function definition is used only if that function is called without that parameter.
Very simple example for understanding it:
The definition:
def print_it( message = "Hello!" ):
print( message)
Calling it without a parameter:
print_it(); # The default value will be used
gives the output
Hello!
Calling it with a parameter
print_it( "Give me your money!" ) # The provided value will be used
gives the output
"Give me your money!"
The default value is in this case totally ignored.
来源：https://stackoverflow.com/questions/44333937/python-how-key-is-passed-into-the-function-with-default-value-of-none