Python的表达评估机制

Question

I used to believe for the Python expression f(g)(x) , the evaluation process is:

1. Evaluate f(g) , return function h ;
2. Evaluate h(x) , return the final result.

But the code below shows this is not true:

def double(x):
    return 2 * x

def double_or_zero(f):
    def f_or_zero(x):
        if x > 0:
            return f(x)
        else:
            return -999
    return f_or_zero

print(double_or_zero(double)(3))
print(double_or_zero(double)(-3))

When the first part (if it exists) double_or_zero(double) is evaluated, it can "see" the parameter ( 3 and -3 ) in the next step, which shows the from-left-to-right evaluation model is not true.

So what is the expression evaluation mechanism for Python (2.x and 3.x)?

Update

Thanks to Bi Rico's answer, Python evaluates expression from left to right strictly. Hope the code below can make it more clear:

def double(x):
    return 2 * x

def f_or_zero(this_f):
    def this_f_or_zero(x):
        print('I am running with x = %s' % x)
        if x > 0:
            return this_f(x)
        else:
            return -999
    print('I am created: %s' % this_f_or_zero)
    return this_f_or_zero

double_or_zero = f_or_zero(double)
print(double_or_zero(3))
print(double_or_zero(-3))

Run it:

$ python demo.py
I am created: <function this_f_or_zero at 0x7f9f53786500>
I am running with x = 3
6
I am running with x = -3
-999

Answer 1

Why do you think that python can see the parameter when the first part is evaluated, it can't. This example might make what's happening more clear. Your example works the same way.

def double(x):
    return 2 * x

def f_or_zero(this_f):
    def this_f_or_zero(x):
        if x > 0:
            return this_f(x)
        else:
            return 0
    return this_f_or_zero


double_or_zero = f_or_zero(double)
print(double_or_zero(3))
print(double_or_zero(-3))

double_or_zero is a function that behaves (mostly) like any other python function. It just so happens that this function is created dynamically and returned by a call to another function.