Why `print` became a function in Python 3

After writing my post on why Python 3 exists which included an explanation about the most common question/complaint about why Python 3 forced textual and binary data into separate types, I heard from people about the second most common question/complaint about Python 3: why did we make print a function?

Who can do what?

The print statement

In its most basic form, print A is the equivalent of sys.stdout.write(str(A) + '\n'). If you pass extra arguments separated by commas then they will also be passed to str() and be printed with a space between each argument. For example, print A, B, C is equivalent to sys.stdout.write(' '.join(map(str, [A, B, C])) + '\n'). If there is a trailing comma then the added newline is left off, e.g. print A, is the same as sys.stdout.write(str(A)).

Introduced in Python 2.0, the print chevron can be used to redirect where the print statement writes the final string. For instance, print >> output, A is the same as output.write(str(A) + '\n').

The print function

The equivalent definition of the print function is:

import sys

def print(*objects, sep=None, end=None, file=None, flush=False):  
    """A Python translation of the C code for builtins.print().


"""
    if sep is None:
        sep = ' '
    if end is None:
        end = '\n'
    if file is None:
        file = sys.stdout
    file.write(sep.join(map(str, objects)) + end)
    if flush:
        file.flush()

As you may have noticed, all the features of the print statement are covered by the print function.

  • print A print(A)
  • print A, B, C print(A, B, C)
  • print A, print(A, end='')
  • print >> output, A print(A, file=output)

The obvious thing the print function gets you that the above examples implicitly demonstrate is the ability to specify a different separator and end string compared to the syntactic print statement.

It's all about flexibility

But the real key to the print function is somewhat subtle and it all has to do with flexibility, both for the users and the Python development team. For users, making print a function lets you use print as an expression, unlike the print statement which can only be used as a statement. As an example, let's say you wanted to print an ellipsis after every line to indicate that more work was to be done. With the print statement you would have two options:

# Manually ...
print A, '...'

# For a reusable solution (which also works with a functional print) ...
def ellipsis_print(*args):  
    for arg in args:
        print arg, '',
    print '...'

But for Python 3, you have much better solutions:

# Manually ...
print(A, end='...\n')

# Multiple reusable solutions that won't work with a syntactic print...
ellipsis_print = lambda *args, **kwargs: print(*args, **kwargs, end='...\n')  
# Or ...
import functools  
ellipsis_print = functools.partial(print, end='...\n')  

In other words, with print as a function it becomes composable while as a statement it isn't. Heck, you can even override the print function by assigning to builtins.print while you can't do that with a statement.

The flexibility that the Python development team gains is being unshackled from having to make the features of print work in a syntactic fashion. For instance, if you wanted to add the same flexibility of being able to specify what string to use as the separator in the print statement, how would you do it? It's a rather tough design problem to solve. But with a function, you just add a new argument and you're done. The richness of Python's function parameters give a greater amount of flexibility than syntax does.

It should also be mentioned that as a general guideline, syntax is reserved for things that are either impossible to do otherwise or there is a clear readability benefit to the syntax. In the case of print, the difference between print A and print(A) is negligible and thus there is no loss in readability. And since we were able to completely replace print with a function there was no loss in functionality, hence why we reduced the amount of syntax in Python 3 by making print a function (along with removing a bunch of other bits of syntax).