Question

I observed a strange behaviour of the __future__ module’s print_function in Python 3.2.

Take, for example this code:

from __future__ import print_function
import sys

print('Enter the base path of the images: ', end='')
path = sys.stdin.readline().strip().strip('"')
if len(path) == 0:
    print("No path entered")
else:
    print(root)
print("\n\nPress ENTER to exit")
exit = sys.stdin.readline()

When the script is run, it appears the console waits for the user to press ENTER before displaying the first print statement.
The output then looks like this:

Enter the base path of the images: No path entered


Press ENTER to exit

Needless to day, displaying an empty prompt to the user leads to a lot of confusion especially since a lot of people are afraid of the black window with white text (Command Prompt).

When the code is changed to this

from __future__ import print_function
import sys

print('\nEnter the base path of the images: ', end='') #line now starts with \n
path = sys.stdin.readline().strip().strip('"')
if len(path) == 0:
    print("No path entered")
else:
    print(path)
print("\n\nPress ENTER to exit")
exit = sys.stdin.readline()

Then the output is as expected (assuming we ignore the preceding empty line):

Enter the base path of the images: c:\
c:\


Press ENTER to exit

When the code is run in python 2.6 however, the first one works as expected (i.e. it displays Enter the base path of the images: before waiting to receive input).

This leads me to ask:
Why do I need to precede the print function with a \n in order to get an output displayed in Python 3.2 while I don’t need the \n when running in Python 2.6?
Could it be that the print_function is implemented differently in the two versions?

Answer 1

You are seeing the effects of line buffering. Flush stdout first (using sys.stdout.flush() for backwards compatibility with Python 2):

print('Enter the base path of the images: ', end='')
sys.stdout.flush()

The print() function in Python 2 is certainly different from the one in Python 3 (where the from __future__ import print_function line is effectively meaningless). In Python 3, I/O has been overhauled and stdout buffering semantics have changed subtly. In Python 2, the sys.stdin.readline() call flushes stdout automatically, in Python 3 this is no longer the case.

If you use the input() function instead of reading from stdin directly, you do not need to flush at all:

msg = 'Enter the base path of the images: '
try:
    # python 2
    path = raw_input(msg)
except NameError:
    # python 3
    path = input(msg)