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The Python print function

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The print function is most likely the first function we encounter when learning Python. That encounter usually looks like print("Hello world!"). After that, we go on to learning more stuff about it like being able to pass any number of arguments or of any type etc. I'm writing this article to give an idea how deep this rabbit hole goes. Turns out, the print function is very powerful. So let's get a coffee, put on a dusty pair of sunglasses and bask in its power!

The Basics

The basic premise of the print function is quite, well, basic. It prints out the given arguments to the standard output.

print("Hello world!")

This prints:

Hello world!

Calling it with multiple arguments:

print("hello", "world")

This prints:

hello world

Notice that the two strings, "hello" and "world" have a space character printed between them. The print function is helpful like that. By default, it places a space between every pair of consecutive arguments to be printed.

It doesn't have to be strings either:

print(42, "is the answer")

This prints:

42 is the answer

Let's look at each of these features in detail and see how they work.

Handling of Multiple Arguments

The print function accepts arbitrary number of arguments to be printed. These arguments can't be keyword-arguments, because that doesn't make much sense. That's not to say the print function doesn't accept any keyword arguments, it does. In fact, the space character that shows up between the arguments to be printed, can be changed by providing the sep= keyword argument.

Let's look at the following examples:

>>> print("the", "world", "is", "a", "cruel", "place")
the world is a cruel place

>>> print("the", "world", "is", "a", "cruel", "place", sep="-")
the-world-is-a-cruel-place

>>> print("the", "world", "is", "a", "cruel", "place", sep="")
theworldisacruelplace

In the first example, we don't explicitly give any value to the sep= keyword argument. So it takes it's default value of the space character " ". In the second example, we set it to the dash character "-" and we can see in the output that the strings are printed joined by dashes.

In the third example, we set the sep= to an empty string so the output is all the words printed consecutively making it a cruel experience to read the text.

The sep= argument can be any string, it doesn't have to be a single character and it can contain newlines and any other shenanigans too.

print("the", "birds", "in", "the", "sky", sep="\n  hammertime\n")

This prints the following mind bogglingly useful output:

the
  hammertime
birds
  hammertime
in
  hammertime
the
  hammertime
sky

Yeah that's a useful trick, but please, consider people's sanity when you do such !@#$.

Handling of non-string types

We know that the print function can handle printing objects of any type, not just strings. But how does that work? The simple answer to this is that print will call str on non-string objects, and print the result of that call.

Let's experiment with this. Consider the following class definition, which has just one method, the __str__. If you are unaware of this method, this is what's called when str is applied on an instance of this class. I won't go into details of that as that's not the topic of this article.

class Tantrum:
    def __str__(self):
        return "awesome __str__ of object %r" % id(self)


print(Tantrum())

The output of running this would be something like (the number in the end would obviously be different if you run this script):

awesome __str__ of object 4508612624

So, what happens if our class doesn't define an implementation for the __str__ method? Let's try that out:

class LazySloth:
    pass


print(LazySloth())

This prints the following output (again, the number in the end would obviously be different for you):

<__main__.LazySloth object at 0x105f327d0>

Turns out that when there's no implementation for __str__, calling str on the instance will still produce some information regarding the instance, which is what we got above.

A neat thing here is that this output is actually what calling repr on the instance would produce. So, it looks like str is falling back to returning the output of repr, when there's no implementation for __str__ provided. Let's confirm this by defining a __repr__ method:

class RatInFormals:
    def __repr__(self):
        return "a sad overridden __repr__ for instance %r" % id(self)


print(RatInFormals())

This prints the following output (again, the number will be different for you):

a sad overridden __repr__ for instance 4313389264

Now we get the output of the overridden __repr__.

So here's how it works. The print function calls str on any non-string objects, which returns the result of the __str__ method, if available, or the result of calling repr on the instance, which in turn returns the result of the __repr__ method, which results in a generic output unless overridden (like in the last example above).

This should be case in favor towards spending a few seconds thinking about and writing useful __str__ methods for your custom types. Someone walking along working with your code later on, might just print an instance of your class to see what's in it, and the generic output with the instance's id is unlikely to be very helpful.

Write to files

Another keyword argument accepted by print is file=. This can be set to a file object, in which case the printing will be done to that file object instead of standard output.

Let's try writing text to a file using the print function like this:

with open("outputs.txt", "w") as f:
    print("Stuff that doesn't show up in standard output", file=f)

Running this script obviously doesn't print anything to the standard output. Instead, a file "outputs.txt" is created which contains the following text:

Stuff that doesn't show up in standard output

Note that since we are opening the file with mode as "w", so if a file named "outputs.txt" already exists in the current folder, it will be overwritten.

Using sys.stderr

The sys.stderr object in the sys module is a file-like object that represents the standard error. Writing to this file-like object directs it to the standard error stream. This is similar to the sys.stdout object which represents the standard output stream, in a similar fashion.

The file= keyword argument can be set to sys.stderr which will print to the standard error stream.

import sys

print("stuff going to standard error", file=sys.stderr)

You might not notice any difference from setting the file= argument in the above script, but if you are running a terminal emulator / shell that shows standard error in red color, then you'll be able to see a difference.

Modifying sys.stdout

If we don't set a value explicitly to the file= argument, the output will be sent to the standard output. There's a small note to that point to be observed. In reality, the output will be sent to the sys.stdout file object. Usually, these two are the same. But, of course, we can set sys.stdout to something else.

Consider the following script which changes the value of sys.stdout, prints something, and then restores the value of sys.stdout to its original value.

import sys

original_stdout = sys.stdout
with open("out.txt", "w") as f:
    sys.stdout = f
    print("trololololol")

sys.stdout = original_stdout
print("restored")

If we run this script, we'll only see restored in the output, but the file out.txt will be created with the output from line 6.

A minor point to note here is that it's probably incorrect to say "the default value of the file argument is sys.stdout". Since if that were the case, changing the value of sys.stdout should not affect the print function. Instead, I believe its default value is None and in that case, print uses the current value of sys.stdout.

We can verify this by explicitly passing in None to the file= argument:

import sys

original_stdout = sys.stdout
with open("out.txt", "w") as f:
    sys.stdout = f
    print("trololololol", file=None)


sys.stdout = original_stdout
print("restored", file=None)

The above script produces the exact same output as when we didn't provide the file= argument explicitly.

Collecting with io.StringIO

The io.StringIO can be used to create a file object that collects all that is written to it, and then get it all out as a string. This is useful when calling a function that prints information using the print function, but instead, we want that output as a string for further processing. We can replace sys.stdout with a io.StringIO instance before calling that function, and then restore it after. Here's how this might look like:

import io, sys

def print_product(a, b):
    print(a * b)


original_stdout = sys.stdout
string_io = io.StringIO()

sys.stdout = string_io
print_product(4, 5)
sys.stdout = original_stdout

result = string_io.getvalue()
print("Result is", result)

In this script, the print_product function prints the result of the multiplication, instead of returning it. So to get the result out of it, we replace sys.stdout with a io.StringIO instance and after calling the print_product function, we get the printed result using the .getvalue() method.

However, note that a similar operation with binary data using io.BytesIO is not possible, since the print function converts all its argument to text before writing to the file.

The end= keyword argument

This is like one of those things that we notice only when it's taken away. The print function appends a newline at the end of the last argument to be printed. Check out the following example:

print("hello on day 1")
print("yeah right on day 2")
print("oh to hell with you on day 3")

The output of this script is the following:

hello on day 1
yeah right on day 2
oh to hell with you on day 3

The output from the three print calls shows up in three separate lines, nice and neat. But we never gave a "\n" in our calls to print. It comes from the default value of the end= argument of the print function. If we set the end= argument to something else, it will replace the newline in the end of the output from a print call.

Check out the following script for example:

print("Doing awesome stuff... ", end="")
# do awesome stuff here
print("done")

This script prints the following output:

Doing awesome stuff... done

The output of the two print calls shows up on the same line, since we suppressed the newline that would've been printed from the first call to print, by setting the end= argument to an empty string. The second call to print will continue this sentence and finish the line by adding a newline at the end.

A Note about Python 2

Python 2 had a print statement, which worked similar to the print function in Python 3, but is not as feature-rich. Additionally, being a statement, it couldn't be used in all the places, for example, within in a lambda expression.

However, Python 2.6 introduced a future import that brought the print function to Python 2. Adding a from __future__ import print_function line at the start of a Python 2 file would disable the print statement in that file and turn print into a function. This can be very useful for when migrating to Python 3.

A Sad Imitation

Here's a sad little imitation of the print function that should behave similar to the builtin in most of the features that have been discussed in this article:

import sys

def sad_print(*args, sep=" ", end="\n", file=None):
    (sys.stdout if file is None else file).write(sep.join(map(str, args)) + end)


sad_print("the answer is", 42)

In this sad_print function, what we are essentially doing is:

  1. Pick sys.stdout if file is None.
  2. Call str on all of the provided arguments.
  3. Join the results of the calls to str using the value of sep.
  4. Concatenate the value of end to the result of above step.
  5. Call write on result of point-1, with the result of the above step.

I'm sure the print builtin does quite a bit more than just this one-liner, but doing this can give us some perspective of how all the different pieces fit in together.

The pprint Function

Python's standard library has a pprint module, with a pprint function that takes one argument, and prints it prettily.

For example, consider the following script:

from pprint import pprint

numbers = [1, 2, 3, 4, 5, 6]

print(numbers)
pprint(numbers)

planets = ["Mercury", "Venus", "Earth", "Mars", "Jupiter", "Saturn", "Uranus", "Neptune", "Pluto"]

print(planets)
pprint(planets)

We are calling print and pprint on the same list of strings. Let's look at the output:

[1, 2, 3, 4, 5, 6]
[1, 2, 3, 4, 5, 6]
['Mercury', 'Venus', 'Earth', 'Mars', 'Jupiter', 'Saturn', 'Uranus', 'Neptune', 'Pluto']
['Mercury',
 'Venus',
 'Earth',
 'Mars',
 'Jupiter',
 'Saturn',
 'Uranus',
 'Neptune',
 'Pluto']

As we can see, the output from pprint is prettified, but only if necessary. In the first case, where we were printing just six numbers, the output was fine as a single line so pprint did not cut it up into several lines. But in the second case, the line ends up too long and it may not be comfortable on small terminal screens. So, it cuts it up.

The pprint module can be useful to prettily print (or formatting) lists and dictionaries. Check out its official documentation for more information.

Conclusion

We may not use all these features of the print functions all the time, but I think it's useful to know that print is not just a function that prints the given string. It's quite a bit more than that; and when we need it, it's there without having to import anything. Thank you for reading!

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About the author

Hello, I am Shrikant! I love programming and quantitative financial topics. I mostly write about Python, JavaScript and Vim following my work and experiences. Thank you for checking out my blog! Say hello!

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