Question

When I try this code in Python 2.7.3:

names = ["Paul", "Mary", "Susan"]
names.sort()

def valuate(string):
    print ord('A')
    return sum(ord(s) for s in string)

i = 1
for name in names:
    print i, name, valuate(name)
    i += 1

I expect the output:

65
1 Mary 409
65
2 Paul 402
65
3 Susan 522

But instead I get:

1 Mary 65
409
2 Paul 65
402
3 Susan 65
522

It seems the print statement outputs the i and name values before calling the function. Is that so? Why?

Answer 1

The surprise you’re encountering is that the print statement prints out each of the expressions it is given before evaluating the next one. That is, a statement like print A, B, C is equivalent to:

print A, # note, the trailing comma suppresses the newline
print B,
print C

As you’d expect from separate statements, A gets written out before B or C is evaluated.

That surprise is perhaps part of the reason that Python 3 has done away with the print statement in favor of a builtin print function which behaves more like you expect (all of its arguments are evaluated before the function runs).

In Python 2, you can get the Python 3 style print if you want using a future import:

from __future__ import print_function