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
__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.
__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
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
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
__str__ method is defined,
print(str(t))) will use the result of
__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))