Python __str__和__add__输出

Question

I have the following example code, however, the output from __add__ doesnt look the same as the __str__ output. I need all output to look exactly like the __str__ output format.

class Foo:
    def __init__(self,x,y=0):
        self.x = x
        self.y = y
    def __str__(self):
        return str(str(self.x/self.y) + "%")
    def __add__(self,other):
        if self.y > 50:
            return str((self.x + other.x)/(self.y+other.y)) + "%"
        else:
            return str((self.x + other.x)/(self.y+other.y)) + "%"

__str__ output looks as such:

>>> a = Foo(1,3)
>>> print(a)
>>> 0.33333%

__add__ output:

>>> Foo(1,3) + Foo(0,0)
>>> '0.33333%'

Why are there single quotes when I used __add__ or __sub__ and not when I use __str__ ?

Answer 1

As mentioned above: Your REPL shows quotes, when the result of the last operation is a string. Your REPL doesn't show quotes, when you print a string (and it supresses the resulting None ).

As a minimal example of this behaviour, you can try this:

Python 3.2.4 (default, May 10 2013, 08:57:38) 
[GCC 4.7.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> a = 'Hello'
>>> a
'Hello'
>>> print(a)
Hello

Twice the same string, once displayed with, once without quotes.

---

Also consider returning an actual instance of Foo in your __add__ method:

class Foo:
    def __init__(self, x, y=0):
        self.x = x
        self.y = y
    def __str__(self):
        return str(self.x / self.y) + "%"
    def __add__(self, other):
        return Foo(self.x + other.x, self.y + other.y)

This way you can do eg Foo(1, 2) + Foo(3, 4) + Foo(5, 6) which you cannot with your code.

Answer 2

This has nothing to do with your custom __add__ or even __str__ methods - this is a feature of the interactive interpreter.

Usually, in a Python script, you'd need to use something like print to display output.

In the interactive however, interpreter there's a handy shortcut: You can enter any expression and it will evaluate it, and print the representation of its result. This is often called a read–eval–print loop (REPL) .

Notice the distinction above: It prints the representation of the result, not the result itself. This is usually (for primitive types and common objects) another Python expression that would recreate the same object.

So, in case of a string a = 'foo' the string is foo , but its representation is 'foo' :

>>> a = 'foo'
>>> print a
foo
>>> a
'foo'

To get the representation of an object, Python calls obj.__repr__ (there's also a shorthand repr(obj) which you can use).