Python Expressions , evaluate foo bar

It's fairly simple: you define a function foo with parameter x, inside of foo is another function called bar, which also has a (separate) parameter x. When you call foo(3), it returns bar(x * 2), which in this case is equivalent to bar(6), which in turn returns x + 1, or in this case, 6 + 1, which equals 7. The syntax is a little weird, because most common languages do not allow proper functions (as opposed to lambdas and anonymous functions) inside of other functions, and even worse since both have the same parameter name. It might be a little clearer if they had different parameter names, like:

def foo(x):
    def bar(y):
        return y + 1
    return bar(x * 2)
 
foo(3)