Python partial function

Recently I came across partial This function , I don't understand , I found out later This is mainly to pass the default value ;

namely ：partial The function of the function is ： Fix some parameters of a function , Returns a new function .

from functools import partial
def multiply(x, y):
return x * y
double = partial(multiply, y=2)
"""
partial Receiver function multiply As a parameter , Fix multiply Parameters of y=2, And return a new function to double;
Be similar to ：
def double(x, y=2):
return multiply(x, y)
"""
print(double(3)) # 6

• If you do not use keyword parameters , Use it directly ：double = partial(multiply, 2), be 2 Yes, it is multiply The leftmost parameter , namely x = 2;（ It will assign values from left to right by default , Multiple values can be passed ）
• call double When the method is used , If no keyword parameter is used , The default value is also from Left to right assignment ;
• If partial In the function Parameters have been specified , call double When the method is used , The value passed in after the parameter needs to be specified ;

from functools import partial
def multiply(x, y):
return x * y
double = partial(multiply,x=1)
# print(double(3)) The direct call will report an error , Parameters need to be specified
# TypeError: multiply() got multiple values for argument 'x'
print(double(y=3)) # 3