Alex summarized well but, surprisingly, was too succinct.
First, let me reiterate the main points in Alex’s post:
- The default implementation is useless (it’s hard to think of one which wouldn’t be, but yeah)
__repr__goal is to be unambiguous
__str__goal is to be readable
__str__uses contained objects’
Default implementation is useless
This is mostly a surprise because Python’s defaults tend to be fairly useful. However, in this case, having a default for
__repr__ which would act like:
return "%s(%r)" % (self.__class__, self.__dict__)
would have been too dangerous (for example, too easy to get into infinite recursion if objects reference each other). So Python cops out. Note that there is one default which is true: if
__repr__ is defined, and
__str__ is not, the object will behave as though
This means, in simple terms: almost every object you implement should have a functional
__repr__ that’s usable for understanding the object. Implementing
__str__ is optional: do that if you need a “pretty print” functionality (for example, used by a report generator).
The goal of
__repr__ is to be unambiguous
Let me come right out and say it — I do not believe in debuggers. I don’t really know how to use any debugger, and have never used one seriously. Furthermore, I believe that the big fault in debuggers is their basic nature — most failures I debug happened a long long time ago, in a galaxy far far away. This means that I do believe, with religious fervor, in logging. Logging is the lifeblood of any decent fire-and-forget server system. Python makes it easy to log: with maybe some project specific wrappers, all you need is a
log(INFO, "I am in the weird function and a is", a, "and b is", b, "but I got a null C — using default", default_c)
But you have to do the last step — make sure every object you implement has a useful repr, so code like that can just work. This is why the “eval” thing comes up: if you have enough information so
eval(repr(c))==c, that means you know everything there is to know about
c. If that’s easy enough, at least in a fuzzy way, do it. If not, make sure you have enough information about
c anyway. I usually use an eval-like format:
"MyClass(this=%r,that=%r)" % (self.this,self.that). It does not mean that you can actually construct MyClass, or that those are the right constructor arguments — but it is a useful form to express “this is everything you need to know about this instance”.
Note: I used
%r above, not
%s. You always want to use
%r formatting character, equivalently] inside
__repr__ implementation, or you’re defeating the goal of repr. You want to be able to differentiate
The goal of
__str__ is to be readable
Specifically, it is not intended to be unambiguous — notice that
str(3)==str("3"). Likewise, if you implement an IP abstraction, having the str of it look like 192.168.1.1 is just fine. When implementing a date/time abstraction, the str can be “2010/4/12 15:35:22”, etc. The goal is to represent it in a way that a user, not a programmer, would want to read it. Chop off useless digits, pretend to be some other class — as long is it supports readability, it is an improvement.
__str__ uses contained objects’
This seems surprising, doesn’t it? It is a little, but how readable would it be if it used their
[moshe is, 3, hello world, this is a list, oh I don't know, containing just 4 elements]
Not very. Specifically, the strings in a container would find it way too easy to disturb its string representation. In the face of ambiguity, remember, Python resists the temptation to guess. If you want the above behavior when you’re printing a list, just
print("[" + ", ".join(l) + "]")
(you can probably also figure out what to do about dictionaries.
__repr__ for any class you implement. This should be second nature. Implement
__str__ if you think it would be useful to have a string version which errs on the side of readability.
Unless you specifically act to ensure otherwise, most classes don’t have helpful results for either:
>>> class Sic(object): pass ... >>> print(str(Sic())) <__main__.Sic object at 0x8b7d0> >>> print(repr(Sic())) <__main__.Sic object at 0x8b7d0> >>>
As you see — no difference, and no info beyond the class and object’s
id. If you only override one of the two…:
>>> class Sic(object): ... def __repr__(self): return 'foo' ... >>> print(str(Sic())) foo >>> print(repr(Sic())) foo >>> class Sic(object): ... def __str__(self): return 'foo' ... >>> print(str(Sic())) foo >>> print(repr(Sic())) <__main__.Sic object at 0x2617f0> >>>
as you see, if you override
__repr__, that’s ALSO used for
__str__, but not vice versa.
Other crucial tidbits to know:
__str__ on a built-on container uses the
__repr__, NOT the
__str__, for the items it contains. And, despite the words on the subject found in typical docs, hardly anybody bothers making the
__repr__ of objects be a string that
eval may use to build an equal object (it’s just too hard, AND not knowing how the relevant module was actually imported makes it actually flat out impossible).
So, my advice: focus on making
__str__ reasonably human-readable, and
__repr__ as unambiguous as you possibly can, even if that interferes with the fuzzy unattainable goal of making
__repr__‘s returned value acceptable as input to