FAQ: __str__ vs __repr__

[Dave Benjamin]

Jan Danielsson wrote:
>   Sorry, but I Just Don't Get It. I did search the 'net, I did read the
> FAQ, but I'm too dumb to understand.

Say we define a string "s" as follows:

 >>> s = 'hello'

If we print "s", we see its string form (__str__):

 >>> print s
hello

However, if we just examine "s", we see its representation (__repr__):

 >>> s
'hello'

This can be verified by calling str() and repr() on the string:

 >>> str(s)
'hello'
 >>> repr(s)
"'hello'"

So, the result of repr() includes quotes, whereas the result of str() 
does not. This has useful properties when the object is part of a more 
complex structure. For instance, these two expressions print out the same:

 >>> [s, s]
['hello', 'hello']
 >>> print [s, s]
['hello', 'hello']

That's because, in either case, the repr() form is used. If Python only 
had str(), we would probably expect str(s) = s, so we'd instead see 
something like this imaginary snippet:

 >>> [s, s]
[hello, hello]
 >>> repr([s, s])
'[hello, hello]'

So, the convention in Python is that repr() returns a string that, when 
evaluated, returns the original object. This is not always easy or 
possible to do, but if you can do it, your objects will print nicely 
when nested within Python's built-in data structures. Python's actual 
behavior is this:

 >>> repr([s, s])
"['hello', 'hello']"

And therefore:

 >>> eval(repr([s, s]))
['hello', 'hello']

Also worth noting is that if you define __repr__, the default behavior 
of __str__ is to delegate to that definition, so if you only want to 
define one, it's often more convenient to just define __repr__.

Dave