Python code evaluation order?

Question

The output of following code is

5
3

I am new to Python, could anybody explain to me why?

import sys

def Main():
     str='1+2'
     print eval(str)

class A:
    def __init__(self):
        self.x = 5

a = A()
print a.x

if __name__=="__main__":
    Main()

On a side-note, see PEP8 (The Python style specification), on function naming conventions. python.org/dev/peps/pep-0008/#function-names — pygeek
– pygeek, Commented Jul 29, 2013 at 5:24

Sukrit Kalra · Accepted Answer · 2013-07-29 04:23:17Z

10

Python code is evaluated from top-down, not from Main().

The interpreter sees the a = A() line first, and prints a.x which is equal to 5, then it checks for the if condition and prints eval(str) which is 3.

Hence the output,

 5
 3

answered Jul 29, 2013 at 4:23

Sukrit Kalra

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3 Comments

jpmc26 Over a year ago

Minor detail. Actually, I believe the interpreter sees the def and class lines first, and it parses the bodies and stores the results in variables. It doesn't evaluate any of the function code while doing so, though.

Sukrit Kalra Over a year ago

@jpmc26 : Yes, you are right. The function definitions are evaluated first, but they are not executed until such time that they are called. :)

jpmc26 Over a year ago

I wasn't sure, so I did some testing. If you add print 'In class A def!' inside A's definition (and outside the __init__ definition, it actually does get printed first. So code inside the class definition but outside its functions gets executed when the class is parsed.

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