Methods __str__, __repr__

Special methods __str__ and __repr__ are responsible for string representation of the object. They are used in different places.

Consider example of IPAddress class that is responsible for representing IPv4 address:

class IPAddress:
    def __init__(self, ip):
        self.ip = ip

After creating class instances, they have a default string view that looks like this (the same output is displayed when print is used):

In [2]: ip1 = IPAddress('10.1.1.1')

In [3]: ip2 = IPAddress('10.2.2.2')

In [4]: str(ip1)
Out[4]: '<__main__.IPAddress object at 0xb4e4e76c>'

In [5]: str(ip2)
Out[5]: '<__main__.IPAddress object at 0xb1bd376c>'

Unfortunately, this presentation is not very informative. It would be better to show information about which address this instance represents. Special method __str__ is responsible for displaying information when using str function. As an argument this method expects only instance and must return string.

class IPAddress:
    def __init__(self, ip):
        self.ip = ip

    def __str__(self):
        return f"IPAddress: {self.ip}"


In [7]: ip1 = IPAddress('10.1.1.1')

In [8]: ip2 = IPAddress('10.2.2.2')

In [9]: str(ip1)
Out[9]: 'IPAddress: 10.1.1.1'

In [10]: str(ip2)
Out[10]: 'IPAddress: 10.2.2.2'

A second string view which is used in Python objects is displayed when using repr function and when adding objects to containers such as lists:

In [11]: ip_addresses = [ip1, ip2]

In [12]: ip_addresses
Out[12]: [<__main__.IPAddress at 0xb4e40c8c>, <__main__.IPAddress at 0xb1bc46ac>]

In [13]: repr(ip1)
Out[13]: '<__main__.IPAddress object at 0xb4e40c8c>'

Method __repr__ is responsible for this output and it should also return a string, but it would return a string by copying which you can get an instance of a class:

class IPAddress:
    def __init__(self, ip):
        self.ip = ip

    def __str__(self):
        return f"IPAddress: {self.ip}"

    def __repr__(self):
        return f"IPAddress('{self.ip}')"


In [15]: ip1 = IPAddress('10.1.1.1')

In [16]: ip2 = IPAddress('10.2.2.2')

In [17]: ip_addresses = [ip1, ip2]

In [18]: ip_addresses
Out[18]: [IPAddress('10.1.1.1'), IPAddress('10.2.2.2')]

In [19]: repr(ip1)
Out[19]: "IPAddress('10.1.1.1')"