Python. Another bracket next to a function

Question

I have code as below.

x = inputs
if conv_first:
    x = conv(x)
    if batch_normalization:
        x = BatchNormalization()(x)
    if activation is not None:
        x = Activation(activation)(x)

Here, I dont understand how x = BatchNormalization()(x) works (just like the same, x = Activation(activation)(x) too). If it was BatchNormalization(x), it would have been easy.

Anyone can explain in a concise way what it is and how it works?

Thank you very much in advance.

The term is parentheses. Please see https://stackoverflow.com/questions/6476825/what-do-double-parentheses-mean-in-a-function-call-e-g-funcstuffstuff — aris, Mar 14 '19 at 01:59

score 4 · Answer 1 · answered Mar 14 '19 at 01:53

4

Both seem to be classes that implement __call__(). Then BatchNormalization() creates an instance and (x) calls .__call__(x) on the instance.

answered Mar 14 '19 at 01:53

Klaus D.

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score 3 · Accepted Answer · answered Mar 14 '19 at 01:58

Not sure that is the case, but the syntax is possible if the first called object returns another function.

Consider this code:

def f(arg):
    print(arg)

def g():
    return f

x = "hi"

g()(x)  # equivalent to f(x), since f is what g returns

Notice thay g() returns the function f without actually executing it, which is why there's no parentheses on g's return statement.

Python. Another bracket next to a function

2 Answers2