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I tried to use the C-function printf() in the python command line on Linux. To make that work, I imported ctypes. My problem is: If I create an object of CDLL to use the printf()-function in a loop, I get a really weird output:

>>> import ctypes
>>> libc = ctypes.CDLL("libc.so.6")
>>> for i in range(10):
...     libc.printf("%d", i)
...
01
11
21
31
41
51
61
71
81
91
>>>

However, when I call this loop inside a function, it works as expected:

>>> import ctypes
>>> libc = ctypes.CDLL("libc.so.6")
>>> def pr():
...     for i in range(10):
...         libc.printf("%d", i)
...     libc.printf("\n")
...
>>> pr()
0123456789
>>>

I can't guess what causes this behavior ...
I'm using Python 2.7.6 on Linux if it matters.

EDIT:

Python version / operating system has no influence to this. See PM 2Ring's answer below for details. On Windows you only have to change the initialization of libc to libc = ctypes.CDLL("msvcrt.dll") where .dll is optional. Another way to get the correct output than calling a function would be storing the return value of printf() in a variable:

>>> import ctypes
>>> libc = ctypes.CDLL("libc.so.6")  # "mscvrt.dll" on windows
>>> for i in range(10):
...     r = libc.printf("%d", i)
...
0123456789>>>

I still prefer the function because you can add a concluding linebreak easier.

Aemyl
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    Actually an easier way to suppress output is to just add a semicolon to the end: `libc.printf("%d", i);` – Ajasja Dec 15 '16 at 08:07

2 Answers2

86

Those extra '1's at the end of each number are the return value from printf, which returns the number of chars that it prints. The return value of a function called in the interactive interpreter is automatically printed (unless it's None).

In fact, the interactive interpreter prints any non-None expression that isn't assigned. And of course it adds a newline to those expressions, which explains why the output in your first code block is on separate lines.

Your pr function doesn't have a return statement, so it returns None, and thus no extra stuff gets printed.

PM 2Ring
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  • And it doesn't get printed in the function, because the function code isn't interactive anymore? – Aemyl Dec 14 '16 at 13:11
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    @Aemyl Basically. You're calling `pr` in the interactive context, so its return value gets printed by the interpreter, but any functions that `pr` calls work normally, otherwise the terminal would get filled with unwanted rubbish. :) – PM 2Ring Dec 14 '16 at 13:13
  • ok, so it doesn't matter which python version / operating system I use? – Aemyl Dec 14 '16 at 13:14
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    @Aemyl No, that's standard on all versions of the standard (CPython) interpreter and on all OSes. There _are_ other Python interpreters / IDEs, but by default they also print the value of non-`None` expressions when running in interactive mode (AFAIK), although they may also print additional information. – PM 2Ring Dec 14 '16 at 13:20
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    Same thing without the printf call: `for i in range(10): 1` – deltab Dec 14 '16 at 16:57
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    Note: `ipython` (a _very_ common alternative interactive interpreter that wraps the CPython interpreter, as one of a bazillion features) is smarter, and doesn't output the result of unassigned expressions in a block. It only outputs for individual expressions that aren't assigned. – ShadowRanger Dec 15 '16 at 02:12
  • Hi, Hope you are doing well there. I have couple of questions about python, can we talk over messenger or something? that would be great if we could))) – dara Apr 20 '19 at 08:17
  • @SardorIslomov Hi. You should visit the [Python chat room](https://chat.stackoverflow.com/rooms/6/python). – PM 2Ring Apr 20 '19 at 08:29
5

While PM 2Ring has answered the question quite well, I think it's worth pointing out that printf's behaviour, or something very close, is available as a python built-in, so it's really quite strange that you'd be using the C library version, unless you're using it to learn how to use ctypes, in which case carry on.

But on the other hand, it's strange to want to use ctypes when you haven't used enough python to know how the REPL works... at the risk of sounding arrogant, as someone with 10 years of python experience, I've never actually had to use ctypes yet.

There are two built-in options for string formatting in python:

print("%d" % (i,))

which is the old style, and very similar to printf, and

print("{:d}".format(i))

which is new to python 3, and is somewhat more powerful. There are questions addressing the differences on this site. Both of the above lines will output identically to libc.printf("%d\n", i). It's also possible to print without the newline.

The CPython implementation of "%" string formatting actually uses sprintf under the hood.

Community
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Amanda Ellaway
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    I agree that there's virtually no point in using C's `printf` in Python, apart from as a learning exercise in using `ctypes`. I guess the fact that it returns the number of characters printed _could_ be handy, but it'd be saner to just use normal Python string formatting & get the `len` of the result before printing. FWIW, the `format` function & method have bee back-ported to Python 2.6 & 2.7. And Python 3 introduces yet another way to format strings: [Literal String Interpolation](https://www.python.org/dev/peps/pep-0498) aka f-strings. – PM 2Ring Dec 16 '16 at 02:50
  • FWIW, I haven't used `ctypes` much, either, but [here](http://stackoverflow.com/a/26878137/4014959) is a program I wrote that uses it to access the OpenSSL crypto library in order to implement a resumable SHA256 hash function. – PM 2Ring Dec 16 '16 at 02:53