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

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()
>>> t
Test()
>>> 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))

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

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}

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