python dict get method runs the second argument even when key is in dict

according to the doc, the get method of a dict can have two argument: key and a value to return if that key is not in the dict. However, when the second argument is a function, it’ll run regardless of whether the key is in the dict or not:

def foo():
    print('foo')

params={'a':1}
print(params.get('a', foo()))
# foo
# 1

As shown above, the key a is in the dict, but foo() still runs. What happens here?

>Solution :

This is a common misconception, the arguments must be evaluated whether or not they are used by the function, for example:

>>> def f(x, y, z): pass
... 
>>> f(print(1), print(2), print(3))
1
2
3

So the print(1), print(2), print(3) statements are executed even if the function f does not use the arguments. You can use try except instead to evaluate foo() only when it is needed:

try:
    x = params['a']
except KeyError:
    x = foo()