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

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


>>> class Test:...     def __repr__(self):...         return "Test()"...     def __str__(self):...         return "member of Test"... >>> t = Test()>>> tTest()>>> print(t)member of Test

The __str__ method is what gets called happens when you print it, and the __repr__ method is what happens when you use the repr() function (or when you look at it with the interactive prompt).

If no __str__ method is given, Python will print the result of __repr__ instead. If you define __str__ but not __repr__, Python will use what you see above as the __repr__, but still use __str__ for printing.


As Chris Lutz mentioned, this is defined by the __repr__ method in your class.

From the documentation of repr():

For many types, this function makes an attempt to return a string that would yield an object with the same value when passed to eval(), otherwise the representation is a string enclosed in angle brackets that contains the name of the type of the object together with additional information often including the name and address of the object. A class can control what this function returns for its instances by defining a __repr__() method.

Given the following class Test:

class Test:    def __init__(self, a, b):        self.a = a        self.b = b    def __repr__(self):        return "<Test a:%s b:%s>" % (self.a, self.b)    def __str__(self):        return "From str method of Test: a is %s, b is %s" % (self.a, self.b)

..it will act the following way in the Python shell:

>>> t = Test(123, 456)>>> t<Test a:123 b:456>>>> print repr(t)<Test a:123 b:456>>>> print(t)From str method of Test: a is 123, b is 456>>> print(str(t))From str method of Test: a is 123, b is 456

If no __str__ method is defined, print(t) (or print(str(t))) will use the result of __repr__ instead

If no __repr__ method is defined then the default is used, which is pretty much equivalent to..

def __repr__(self):    return "<%s instance at %s>" % (self.__class__.__name__, id(self))


A generic way that can be applied to any class without specific formatting could be done as follows:

class Element:    def __init__(self, name, symbol, number):        self.name = name        self.symbol = symbol        self.number = number    def __str__(self):        return str(self.__class__) + ": " + str(self.__dict__)

And then,

elem = Element('my_name', 'some_symbol', 3)print(elem)

produces

__main__.Element: {'symbol': 'some_symbol', 'name': 'my_name', 'number': 3}


matomo