Calling function defined in python class. How function are called?

Question

class Vector:
    def __init__(self, x=0, y=0):
        self.x = x
        self.y = y
    
    def __repr__(self):
        return 'Vector(%r, %r)' % (self.x, self.y)

v=Vector(2, 3)
print(v)

In above case: how is repr being called when I print v?

`__repr__` is a hook used by the `repr` function, which is implicitly called by `print` when you print an objext — juanpa.arrivillaga, Jul 22 '20 at 09:21
What do you mean *"how"*? That's what's defined by the interpreter; when you `print` something, it needs to get a printable form. — jonrsharpe, Jul 22 '20 at 09:22

score 0 · Answer 1 · answered Jul 22 '20 at 09:21

The __repr__ method is called by the builtin function repr and is what is echoed on your python shell when it evaluates an expression that returns an object. When you will just print the v variable it will print the return of that function. When you will not include the __repr__ you will get <__main__.Vector object at 0x037B2CA0> where at is thee place in memory

score 0 · Answer 2 · answered Jul 22 '20 at 09:23

repr is a special built-in function of python. Basically, whenever python finds this method within the context of an object, it'll print what this function returns when you attempt to print the object as its "representation"!

Built-in functions can commonly be identified by the double underscores on each side.

You can find the other built-in functions here.

score 0 · Answer 3 · answered Jul 22 '20 at 09:24

This can realize your idea

class Vector:
    def __init__(self, x=0, y=0):
        self.x = x
        self.y = y

    def __str__(self):
        return self.__repr__()

    def __repr__(self):
        return 'Vector(%r, %r)' % (self.x, self.y)

v=Vector(2, 3)
print(v)

And you can learn more from this answer: Difference between __str__ and __repr__?

irudyak · Accepted Answer · 2020-07-22T09:56:09.080

You may call this method explicitly like this:

In [2]: v.__repr__()                                                                                                    
Out[2]: 'Vector(2, 3)' 

In [3]: print(v.__repr__())                                                                                             
Vector(2, 3)

In [4]: type(v.__repr__())                                                                                              
Out[4]: str

The main idea behind this - define a method that is called implicitly when you call print(v).

There's actually one more function in between (so called build-in function) - repr (see here). If you don't define __repr__ it will print some default values.

In [1]: class Vector: 
   ...:     def __init__(self, x=0, y=0): 
   ...:         self.x = x 
   ...:         self.y = y 
   ...:                                                                                                                 

In [2]: v = Vector(2, 3)                                                                                                

In [3]: print(v)                                                                                                        
<__main__.Vector object at 0x108e97a60>

So you may control what it returns:

A class can control what this function returns for its instances by defining a __repr__() method.

Calling function defined in python class. How function are called?

4 Answers4