Understanding Python's interactive mode

The Python interepreter has this convenient feature that when you type something simple onto the interpreter, it spits that value back out at you, so you can copy/paste it back as a value.

But with convenience often comes complexity, let’s look at how this feature works!

How does the print function work?

Before we can talk about typing stuff onto the interpreter, let’s talk about the simpler function print. Let’s take a look at a few examples:

>>> print('abcdef')
abcdef
>>> print('1')
1
>>> print(1)
1
>>> x = print(1)
1
>>> x
>>>

So how does print work? Clearly, it returns None once it’s done. But what’s it doing when it’s printing to the screen. Clearly, something strange is happening where both the string '1' and the number 1 get printed out as the single character 1.

To properly understand print, we need to break it down into its constituent parts. Imagine we have a function print_string, which takes a string and prints it to a terminal. Then, we have the following behavior:

>>> print_string('abcdef')
abcdef
>>> print_string('1')
1
>>> print_string(1)
Error

Which is somewhat more consistent. We can implement print_string in terms of print fairly easily as follows:

def print(x):
    print_string(str(x))

where str is a function that takes in a value and outputs some string. We have the following behavior for str (illustrated using lists for reasons that will become clear later).

In the case of a string, it does nothing, and in the case of an integer, it converts it into a string. We’ll come back to str later, but for now think of it as providing a nice human-readable version of the given object.

repr

But if we want an output that isn’t human-readable, but is instead computer-readable, we can use repr instead. For example, repr(1) and repr('1') should produce different outputs as they should be read as two different types. Let’s look at some examples:

>>> print_string(repr('abcdef'))
'abcdef'
>>> print_string(repr('1'))
'1'
>>> print_string(repr(1))
1

Note for later: print_string(repr(<blah>)) prints out <blah>

Looking at an the same example as before using an environment diagram, we have the same result for repr(12) as str(12) but repr('12') has a set of quotes around it while str('12') does not.

This difference can be attributed to the fact that typing in 12 gets us 12, while we need to type in '12' to get '12', whereas 12 appropriately represents both concepts to a human reader.

Typing something into the interpreter

In the last section, we saw how the repr function works, especially on strings, where it adds a layer of quotes. You might notice that typing something directly onto the interpreter performs a similar task. That’s because it more or less is. We can consider typing something directly into the interpreter (>>> x) to be the same as calling >>> print_string(repr(x)). The examples from above have the same behavior:

>>> 'abcdef'
'abcdef'
>>> '1'
'1'
>>> 1
1

In fact, this is the motivation behind repr, if you type something into the interpreter, it spits it back out at you.

Typing repr(something) into the interpeter

Let’s say you did the following:

>>> repr('hi')
"'hi'"

What happened? We now have two sets of quotes around the hi. Did the repr function add two sets of quotes instead of one? The answer is no, because, as you can recall from the previous section, that example is equivalent to

>>> print_string(repr(repr('hi')))
"'hi'"

Using our environment diagram, we can see the following:

We see that repr('hi') has four characters in it: ', h, i, and '. repr(repr('hi')) needs to represent this, so it has six characters in it: ", ', h, i, ', ". These six characters get printed to the terminal, to form the string "'hi'".

__str__ and __repr__

OK, so now that we know that str, repr, print, and typing something directly into the terminal do, let’s look at how these concepts apply to user-defined functions. Take the following class:

class A:
    def __repr__(self):
        return "hi"
    def __str__(self):
        return "bye!"

The idea is that whenever we call str(x) when x is of type A, this is equivalent to calling x.__str__(), and whenever we call repr(x) when x is of type A, this is equivalent to calling x.__repr__().

Examples

Here are some examples of how custom __str__ and __repr__ methods work in practice. Before we get into the explanations, try to figure them out on your own

>>> a = A()
>>> a
hi
>>> print(a)
bye!
>>> repr(a)
'hi'
>>> str(a)
'bye!'

Your first question might be: why no parentheses when we type a into the terminal? The answer is that >>> a is equivalent to print_string(repr(a)), and we have that repr(a) is the string of length 2 containing the letters h, and i; which then gets printed. On the other hand, >>> repr(a) is equivalent to print_string(repr(repr(a))), and repr(repr(a)) is the string of length 4 containing the letters ', h, i, and '. The following environment diagram describes how this process happens: