How to print instances of a class using print()?

When I try to print an instance of a class, I get an output like this:

>>> class Test():
...     def __init__(self):
...         self.a = 'foo'
...
>>> print(Test())
<__main__.Test object at 0x7fc9a9e36d60>

How can I make it so that the print will show something custom (e.g. something that includes the a attribute value)? That is, how can I can define how the instances of the class will appear when printed (their string representation)?

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_{See How can I choose a custom string representation for a class itself (not instances of the class)? if you want to define the behaviour for the class itself (in this case, so that print(Test) shows something custom, rather than or similar). (In fact, the technique is essentially the same, but trickier to apply.)}