Implementation of 'print' in Python

Question

I was just curious as to how built-in function ‘print’ might be working behind the scenes in Python3. So, the following code snippet is my attempt to write a print function of my own but I’m not sure if it accurately represents how the actual ‘print’ works:

import os
import sys
def my_print(*args, **kwargs):
    sep = kwargs.get('sep', ' ')
    end = kwargs.get('end', os.linesep)
    if end is None:
        end = os.linesep
    file = kwargs.get('file', sys.stdout)
    flush = kwargs.get('flush', False)
    file.write('%s%s' % (sep.join(str(arg) for arg in args), end))
    if flush:
        file.flush()

I would appreciate if anyone who knows how built-in ‘print’ works, assess the accuracy of my version and point out any deficiency.

Asked By: deniz

||

Source

Answer 1

print is a built-in function in Python 3. Most built-in functions are implemented in C (in the default CPython interpreter anyway), and print is no exception. The implementation is builtin_print_impl in Python/bltinmodule.c, which can be seen here: https://github.com/python/cpython/blob/v3.11.2/Python/bltinmodule.c#L1986

The PyPy interpreter, on the other hand, is implemented in a subset of Python, so it has a print function written in Python in pypy/module/__builtin__/app_io.py, which can be seen here: https://foss.heptapod.net/pypy/pypy/-/blob/release-pypy3.9-v7.3.11/pypy/module/__builtin__/app_io.py#L87

Here’s the relevant code; it’s fairly short:

def print_(*args, sep=' ', end='n', file=None, flush=False):
    r"""print(value, ..., sep=' ', end='n', file=sys.stdout, flush=False)

    Prints the values to a stream, or to sys.stdout by default.
    Optional keyword arguments:
    file:  a file-like object (stream); defaults to the current sys.stdout.
    sep:   string inserted between values, default a space.
    end:   string appended after the last value, default a newline.
    flush: whether to forcibly flush the stream.
    """
    fp = file
    if fp is None:
        fp = sys.stdout
        if fp is None:
            return
    if sep is None:
        sep = ' '
    if not isinstance(sep, str):
        raise TypeError("sep must be None or a string")
    if end is None:
        end = 'n'
    if not isinstance(end, str):
        raise TypeError("end must be None or a string")
    if len(args) == 1:
        fp.write(str(args[0]))
    else:
        for i, arg in enumerate(args):
            if i:
                fp.write(sep)
            fp.write(str(arg))
    fp.write(end)
    if flush:
        fp.flush()

Answered By: nneonneo

Implementation of 'print' in Python

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