Pythons str vs repr

The goal of __repr__ is to be unambiguous and the goal of __str__ is to be readable. Bonus points, if __repr__ returns what is needed to create the object. Some examples are:

>>> repr([42, 1337])
'[42, 1337]'
>>> repr({42, 1337})
'{1337, 42}'

>>> from collections import namedtuple
>>> Location = namedtuple('Location', ['lon', 'lat'])
>>> a = Location(12, 34)
>>> repr(a)
'Location(lon=12, lat=34)'

>>> import re; pattern = re.compile('foo')
>>> repr(pattern)
"re.compile('foo')"

In all of those cases the unambiguous representation and the readable string are the same.

Where it's used

The following code sample shows how __repr__ and __str__ are used:

class Foo(object):

    def __repr__(self):
        return '__repr__'

    def __str__(self):
        return '__str__'


if __name__ == '__main__':
    bar = Foo()

    print(bar)
    print(str(bar))
    print(repr(bar))

Playing with this gives the following insights:

• If neither __repr__ nor __str__ is implemented, then bar is printed as <__main__.Foo object at 0x7f42319379e8>
• If either of them is implemented, then print(bar) uses that one.
• If both of them are implemented, then print(bar) uses __str__.
• The REPL uses __repr__.

3rd Party Libraries

It's a bit different once you get to third party libraries:

>>> import numpy as np
>>> a = np.array([42, 1337])
>>> str(a)
'[  42 1337]'
>>> repr(a)
'array([  42, 1337])'

Some - like Pandas Dataframes have a very different string representation.

Others just don't care to implement somehting reasonable at all:

>>> from keras.models import Sequential
>>> model = Sequential()
>>> repr(model)
'<keras.engine.sequential.Sequential object at 0x7f8d86b8eb70>'

Enums

Also - sadly - it is not done well with Enums:

class ExistsStrategy(enum.Enum):
    """Strategies what to do when a file already exists."""

    RAISE = 'raise'
    REPLACE = 'replace'
    ABORT = 'abort'

print(str(ExistsStrategy.RAISE))
print(repr(ExistsStrategy.RAISE))

The str gives ExistsStrategy.RAISE which is awesome. The repr gives <ExistsStrategy.RAISE: 'raise'>. I don't understand why it doesn't give the same result as the str representation.

See also

• Difference between __str__ and __repr__?