Using print() inside recursive functions in Python3

Question

I am following the book Introduction to Computing Using Python, by Ljubomir Perkovic, and I am having trouble with one of the examples in recursion section of the book. The code is as follows:

def pattern(n):
    'prints the nth pattern'
    if n == 0:    # base case
        print(0, end=' ')
    else:    #recursive step: n > 0
        pattern(n-1)         # print n-1st pattern
        print(n, end=' ')    # print n
        pattern(n-1)         # print n-1st pattern

For, say, pattern(1), the output should be 0 1 0, and it should be displayed horizontally. When calling the function pattern(1), nothing prints out, however. But if this is followed by a print statement without arguments, then the results are displayed.

>>>pattern(1)
>>>print()
0 1 0

If I remove the end argument of the print() functions inside the recursive function, I get correct output (albeit it displays it vertically):

>>> pattern(1)
0
1
0

This makes me think that the recursive code itself is correct (plus I confirmed it was with the source provided by the book's website, and with the errata sheet). I am not sure, however, why the print statement isn't printing the output as the functions run, if the end parameter is included. Any help would be greatly appreciated.

This problem may be IDE-specific. It does not seem to occur using Python3 run on Ubuntu/gnome-terminal (or in IPython+emacs). — unutbu
– unutbu, Commented Apr 25, 2013 at 20:53

Bakuriu · Accepted Answer · 2013-04-25 20:45:50Z

8

The print function doesn't always flush the output. You should flush it explicitly:

import sys

def pattern(n):
    'prints the nth pattern'
    if n == 0:    # base case
        print(0, end=' ')
    else:    #recursive step: n > 0
        pattern(n-1)         # print n-1st pattern
        print(n, end=' ')    # print n
        pattern(n-1)         # print n-1st pattern
    sys.stdout.flush()

Note that on python3.3 print has a new keyword argument flush that you can use to forcibly flush the output(and thus avoid using sys.stdout.flush).

On a general note I'd decouple the output from the pattern, doing, for example:

def gen_pattern(n):
    if n == 0:
        yield 0
    else:
        for elem in gen_pattern(n-1):
            yield elem
        yield n
        for elem in gen_pattern(n-1):
            yield elem

def print_pattern(n):
    for elem in gen_pattern(n):
        print(elem, end=' ')
     sys.stdout.flush()

This makes the code more flexible and reusable, and has the advantage of calling flush only once, or you could also call it once every x elements(actually I believe print already does this. It flushes if trying to write many characters on the screen).

In python3.3 the code could be simplified a little:

def gen_pattern(n):
    if n == 0:
        yield 0
    else:
        yield from gen_pattern(n-1)
        yield n
        yield from gen_pattern(n-1)

edited Apr 25, 2013 at 20:45

answered Apr 25, 2013 at 20:36

Bakuriu

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2 Comments

Lev Levitsky Over a year ago

But not flushing in the interactive shell seems very strange.

Bakuriu Over a year ago

@LevLevitsky Not if you do something like pattern(1); time.sleep(10)

Ashwini Chaudhary · Accepted Answer · 2013-04-25 20:52:11Z

5

The reason is that when end is used with some value other than a "\n" then the print function accumulates the whole value and prints the output only when a newline is to be printed or the loop is over.

See the difference in these two programs:

In [17]: for x in range(5):
    print(x,end=" ")
    if x==3:
       print(end="\n")
    sleep(2)
   ....:     
0 1 2 3  #first this is printed
4        #and then after a while this line is printed
In [18]: for x in range(5):
    print(x,end=" ")
    if x==3:
       print(end="\t")
    sleep(2)
   ....:     
0 1 2 3     4   #whole line is printed at once

edited Apr 25, 2013 at 20:52

answered Apr 25, 2013 at 20:42

Ashwini Chaudhary

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