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?
It's not just the print statement, it's the function call.
print i, name, valuate(name)
Prints i, then name, then calls valuate which prints 65 (and a new line) then on return the print statement continues to print the return value (and another newline).
The sequence of events is as follows:
print i,
print name,
val = valuate(name) # prints ord('A')
print val
This is confirmed by looking at the bytecodes generated for print i, name, valuate(name):
11 19 LOAD_FAST 0 (i)
22 PRINT_ITEM
23 LOAD_FAST 1 (name)
26 PRINT_ITEM
27 LOAD_GLOBAL 1 (valuate)
30 LOAD_FAST 1 (name)
33 CALL_FUNCTION 1
36 PRINT_ITEM
37 PRINT_NEWLINE
I don't know if this evaluation order is guaranteed (a quick search didn't reveal anything). I definitely wouldn't rely on it, and therefore wouldn't write code like this.
It's very simple. Your print statement evaluates the arguments lazily. It prints i first, then name, and then it calls valuate. Valuate prints 65. Then your print statement prints the result of valuate.
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
I think that a small ',' would make things a lot more simple to grasp.
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
Now that outputs:
1 Mary 65 409
2 Paul 65 402
3 Susan 65 522
The comma after print makes it not to print a new line character at the end. So now you can see that valuate first prints the ord('A') and then returns the output for the outer print. (I also indented the i += 1 line)
printbecame a function, it works the way this OP expected.