How to print a class or objects of class using print()?
I am learning the ropes in Python. When I try to print an object of class Foobar
using the print()
function, I get an output like this:
<__main__.Foobar instance at 0x7ff2a18c>
Is there a way I can set the printing behaviour (or the string representation) of a class and its objects? For instance, when I call print()
on a class object, I would like to print its data members in a certain format. How to achieve this in Python?
If you are familiar with C++ classes, the above can be achieved for the standard ostream
by adding a friend ostream& operator << (ostream&, const Foobar&)
method for the class.
>>> 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 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 this isn't the most Pythonic method, I apologize, because I'm still learning too - but it works.
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}
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