What is the difference between __str__ and __repr__ in Python?
3067
votes
24 Answers
3011
votes
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- Container’s
__str__uses contained objects’__repr__
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 __str__=__repr__.
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 repr() [or %r formatting character, equivalently] inside __repr__ implementation, or you’re defeating the goal of repr. You want to be able to differentiate MyClass(3) and MyClass("3").
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.
Container’s __str__ uses contained objects’ __repr__
This seems surprising, doesn’t it? It is a little, but how readable would it be if it used their __str__?
[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.
Summary
Implement __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.
444
votes
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__(object): return 'foo'
...
>>> print str(Sic())
foo
>>> print repr(Sic())
foo
>>> class Sic(object):
... def __str__(object): 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 __eval__!
178
votes
__repr__: representation of python object usually eval will convert it back to that object
__str__: is whatever you think is that object in text form
e.g.
>>> s="""w'o"w"""
>>> repr(s)
'\'w\\\'o"w\''
>>> str(s)
'w\'o"w'
>>> eval(str(s))==s
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<string>", line 1
w'o"w
^
SyntaxError: EOL while scanning single-quoted string
>>> eval(repr(s))==s
True
169
votes
In short, the goal of
__repr__is to be unambiguous and__str__is to be readable.
Here is a good example:
>>> import datetime
>>> today = datetime.datetime.now()
>>> str(today)
'2012-03-14 09:21:58.130922'
>>> repr(today)
'datetime.datetime(2012, 3, 14, 9, 21, 58, 130922)'
Read this documentation for repr:
repr(object)Return a string containing a printable representation of an object. This is the same value yielded by conversions (reverse quotes). It is sometimes useful to be able to access this operation as an ordinary function. 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__repr__()method.
Here is the documentation for str:
str(object='')Return a string containing a nicely printable representation of an object. For strings, this returns the string itself. The difference with
repr(object)is thatstr(object)does not always attempt to return a string that is acceptable toeval(); its goal is to return a printable string. If no argument is given, returns the empty string,''.
133
votes
What is the difference between
__str__and__repr__in Python?
__str__ (read as "dunder (double-underscore) string") and __repr__ (read as "dunder-repper" (for "representation")) are both special methods that return strings based on the state of the object.
__repr__ provides backup behavior if __str__ is missing.
So one should first write a __repr__ that allows you to reinstantiate an equivalent object from the string it returns e.g. using eval or by typing it in character-for-character in a Python shell.
At any time later, one can write a __str__ for a user-readable string representation of the instance, when one believes it to be necessary.
__str__
If you print an object, or pass it to format, str.format, or str, then if a __str__ method is defined, that method will be called, otherwise, __repr__ will be used.
__repr__
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.
Since it provides a backup for __str__, if you can only write one, start with __repr__
Here's the builtin help on repr:
repr(...)
repr(object) -> string
Return the canonical string representation of the object.
For most object types, eval(repr(object)) == object.
That is, for most objects, if you type in what is printed by repr, you should be able to create an equivalent object. But this is not the default implementation.
Default Implementation of __repr__
The default object __repr__ is (C Python source) something like:
def __repr__(self):
return '<{0}.{1} object at {2}>'.format(
self.__module__, type(self).__name__, hex(id(self)))
That means by default you'll print the module the object is from, the class name, and the hexadecimal representation of its location in memory - for example:
<__main__.Foo object at 0x7f80665abdd0>
This information isn't very useful, but there's no way to derive how one might accurately create a canonical representation of any given instance, and it's better than nothing, at least telling us how we might uniquely identify it in memory.
How can __repr__ be useful?
Let's look at how useful it can be, using the Python shell and datetime objects. First we need to import the datetime module:
import datetime
If we call datetime.now in the shell, we'll see everything we need to recreate an equivalent datetime object. This is created by the datetime __repr__:
>>> datetime.datetime.now()
datetime.datetime(2015, 1, 24, 20, 5, 36, 491180)
If we print a datetime object, we see a nice human readable (in fact, ISO) format. This is implemented by datetime's __str__:
>>> print(datetime.datetime.now())
2015-01-24 20:05:44.977951
It is a simple matter to recreate the object we lost because we didn't assign it to a variable by copying and pasting from the __repr__ output, and then printing it, and we get it in the same human readable output as the other object:
>>> the_past = datetime.datetime(2015, 1, 24, 20, 5, 36, 491180)
>>> print(the_past)
2015-01-24 20:05:36.491180
How do I implement them?
As you're developing, you'll want to be able to reproduce objects in the same state, if possible. This, for example, is how the datetime object defines __repr__ (Python source). It is fairly complex, because of all of the attributes needed to reproduce such an object:
def __repr__(self):
"""Convert to formal string, for repr()."""
L = [self._year, self._month, self._day, # These are never zero
self._hour, self._minute, self._second, self._microsecond]
if L[-1] == 0:
del L[-1]
if L[-1] == 0:
del L[-1]
s = "%s.%s(%s)" % (self.__class__.__module__,
self.__class__.__qualname__,
", ".join(map(str, L)))
if self._tzinfo is not None:
assert s[-1:] == ")"
s = s[:-1] + ", tzinfo=%r" % self._tzinfo + ")"
if self._fold:
assert s[-1:] == ")"
s = s[:-1] + ", fold=1)"
return s
If you want your object to have a more human readable representation, you can implement __str__ next. Here's how the datetime object (Python source) implements __str__, which it easily does because it already has a function to display it in ISO format:
def __str__(self):
"Convert to string, for str()."
return self.isoformat(sep=' ')
Set __repr__ = __str__?
This is a critique of another answer here that suggests setting __repr__ = __str__.
Setting __repr__ = __str__ is silly - __repr__ is a fallback for __str__ and a __repr__, written for developers usage in debugging, should be written before you write a __str__.
You need a __str__ only when you need a textual representation of the object.
Conclusion
Define __repr__ for objects you write so you and other developers have a reproducible example when using it as you develop. Define __str__ when you need a human readable string representation of it.
44
votes
On page 358 of the book Python scripting for computational science by Hans Petter Langtangen, it clearly states that
- The
__repr__aims at a complete string representation of the object; - The
__str__is to return a nice string for printing.
So, I prefer to understand them as
- repr = reproduce
- str = string (representation)
from the user's point of view although this is a misunderstanding I made when learning python.
A small but good example is also given on the same page as follows:
Example
In [38]: str('s')
Out[38]: 's'
In [39]: repr('s')
Out[39]: "'s'"
In [40]: eval(str('s'))
Traceback (most recent call last):
File "<ipython-input-40-abd46c0c43e7>", line 1, in <module>
eval(str('s'))
File "<string>", line 1, in <module>
NameError: name 's' is not defined
In [41]: eval(repr('s'))
Out[41]: 's'
43
votes
Apart from all the answers given, I would like to add few points :-
1) __repr__() is invoked when you simply write object's name on interactive python console and press enter.
2) __str__() is invoked when you use object with print statement.
3) In case, if __str__ is missing, then print and any function using str() invokes __repr__() of object.
4) __str__() of containers, when invoked will execute __repr__() method of its contained elements.
5) str() called within __str__() could potentially recurse without a base case, and error on maximum recursion depth.
6) __repr__() can call repr() which will attempt to avoid infinite recursion automatically, replacing an already represented object with ....
15
votes
To put it simply:
__str__ is used in to show a string representation of your object to be read easily by others.
__repr__ is used to show a string representation of the object.
Let's say I want to create a Fraction class where the string representation of a fraction is '(1/2)' and the object (Fraction class) is to be represented as 'Fraction (1,2)'
So we can create a simple Fraction class:
class Fraction:
def __init__(self, num, den):
self.__num = num
self.__den = den
def __str__(self):
return '(' + str(self.__num) + '/' + str(self.__den) + ')'
def __repr__(self):
return 'Fraction (' + str(self.__num) + ',' + str(self.__den) + ')'
f = Fraction(1,2)
print('I want to represent the Fraction STRING as ' + str(f)) # (1/2)
print('I want to represent the Fraction OBJECT as ', repr(f)) # Fraction (1,2)
13
votes
In all honesty, eval(repr(obj)) is never used. If you find yourself using it, you should stop, because eval is dangerous, and strings are a very inefficient way to serialize your objects (use pickle instead).
Therefore, I would recommend setting __repr__ = __str__. The reason is that str(list) calls repr on the elements (I consider this to be one of the biggest design flaws of Python that was not addressed by Python 3). An actual repr will probably not be very helpful as the output of print [your, objects].
To qualify this, in my experience, the most useful use case of the repr function is to put a string inside another string (using string formatting). This way, you don't have to worry about escaping quotes or anything. But note that there is no eval happening here.
12
votes
From an (An Unofficial) Python Reference Wiki (archive copy) by effbot:
__str__ "computes the "informal" string representation of an object. This differs from __repr__ in that it does not have to be a valid Python expression: a more convenient or concise representation may be used instead."
11
votes
8
votes
One aspect that is missing in other answers. It's true that in general the pattern is:
- Goal of
__str__: human-readable - Goal of
__repr__: unambiguous, possibly machine-readable viaeval
Unfortunately, this differentiation is flawed, because the Python REPL and also IPython use __repr__ for printing objects in a REPL console (see related questions for Python and IPython). Thus, projects which are targeted for interactive console work (e.g., Numpy or Pandas) have started to ignore above rules and provide a human-readable __repr__ implementation instead.
7
votes
7
votes
Q: What's the difference between __str__() and __repr__()?
TL;DR:
LONG
This question has been around a long time, and there are a variety of answers of which most are correct (not to mention from several Python community legends[!]). However when it comes down to the nitty-gritty, this question is analogous to asking for the difference between the str() and repr() built-in functions. I'm going to describe the differences in my own words (which means I may be "borrowing" liberally from Core Python Programming so pls forgive me).
Both str() and repr() have the same basic job: their goal is to return a string representation of a Python object. What kind of string representation is what differentiates them.
str()&__str()__return a printable string representation of an object... something human-readable/for human consumptionrepr()&__repr()__return a string representation of an object that is a valid Python object, something you can pass toeval()or type into the Python shell without getting an error.
For example, let's assign a string to x and an int to y, and simply showing human-readable string versions of each:
>>> x, y = 'foo', 123
>>> str(x), str(y)
('foo', '123')
Can we take what's inside the quotes in both cases and enter them verbatim into the Python interpreter? Let's give it a try:
>>> 123
123
>>> foo
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'foo' is not defined
Clearly you can for an int but not necessarily for a str. While I can pass '123' to eval(), that doesn't work for 'foo':
>>> eval('123')
123
>>> eval('foo')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<string>", line 1, in <module>
NameError: name 'foo' is not defined
Now let's try repr(); again, dump what's in the pair of quotes for each string:
>>> repr(x), repr(y)
("'foo'", '123')
>>> 123
123
>>> 'foo'
'foo'
Wow, they both work? That's because 'foo', while a printable string representation of that string, it's not evaluatable, but "'foo'" is. 123 is a valid Python int called by either str() or repr(). What happens when we call eval() with these?
>>> eval('123')
123
>>> eval("'foo'")
'foo'
It works because 123 and 'foo' are valid Python objects. Another key takeaway is that while sometimes both return the same thing (the same string representation), that's not always the case. (And yes, yes, I can go create a variable foo where the eval() works, but that's not the point.)
More factoids about both pairs
- Sometimes,
str()andrepr()are called implicitly, meaning they're called on behalf of users: when users executeprint(Py1/Py2) or callprint()(Py3+), even if users don't callstr()explicitly, such a call is made on their behalf before the object is displayed. - In the Python shell (interactive interpreter), if you enter a variable at the
>>>prompt and press RETURN, the interpreter displays the results ofrepr()implicitly called on that object. - To connect
str()andrepr()to__str__()and__repr__(), realize that calls to the built-in functions, i.e.,str(x)orrepr(y)result in calling their object's corresponding special methods:x.__str__()ory.__repr()__ - By implementing
__str__()and__repr__()for your Python classes, you overload the built-in functions (str()andrepr()), allowing instances of your classes to be passed in tostr()andrepr(). When such calls are made, they turn around and call the class'__str__()and__repr__()(per #3).
5
votes
Excellent answers already cover the difference between __str__ and __repr__, which for me boils down to the former being readable even by an end user, and the latter being as useful as possible to developers. Given that, I find that the default implementation of __repr__ often fails to achieve this goal because it omits information useful to developers.
For this reason, if I have a simple enough __str__, I generally just try to get the best of both worlds with something like:
def __repr__(self):
return '{0} ({1})'.format(object.__repr__(self), str(self))
4
votes
One important thing to keep in mind is that container's
__str__uses contained objects'__repr__.
>>> from datetime import datetime
>>> from decimal import Decimal
>>> print (Decimal('52'), datetime.now())
(Decimal('52'), datetime.datetime(2015, 11, 16, 10, 51, 26, 185000))
>>> str((Decimal('52'), datetime.now()))
"(Decimal('52'), datetime.datetime(2015, 11, 16, 10, 52, 22, 176000))"
Python favors unambiguity over readability, the __str__ call of a tuple calls the contained objects' __repr__, the "formal" representation of an object. Although the formal representation is harder to read than an informal one, it is unambiguous and more robust against bugs.
4
votes
In a nutshell:
class Demo:
def __repr__(self):
return 'repr'
def __str__(self):
return 'str'
demo = Demo()
print(demo) # use __str__, output 'str' to stdout
s = str(demo) # __str__ is used, return 'str'
r = repr(demo) # __repr__ is used, return 'repr'
import logging
logger = logging.getLogger(logging.INFO)
logger.info(demo) # use __str__, output 'str' to stdout
from pprint import pprint, pformat
pprint(demo) # use __repr__, output 'repr' to stdout
result = pformat(demo) # use __repr__, result is string which value is 'str'
4
votes
>>> print(decimal.Decimal(23) / decimal.Decimal("1.05"))
21.90476190476190476190476190
>>> decimal.Decimal(23) / decimal.Decimal("1.05")
Decimal('21.90476190476190476190476190')
When print() is called on the result of decimal.Decimal(23) / decimal.Decimal("1.05") the raw number is printed; this output is in string form which can be achieved with __str__(). If we simply enter the expression we get a decimal.Decimal output — this output is in representational form which can be achieved with __repr__(). All Python objects have two output forms. String form is designed to be human-readable. The representational form is designed to produce output that if fed to a Python interpreter would (when possible) reproduce the represented object.
4
votes
__str__ can be invoked on an object by calling str(obj) and should return a human readable string.
__repr__ can be invoked on an object by calling repr(obj) and should return internal object (object fields/attributes)
This example may help:
class C1:pass
class C2:
def __str__(self):
return str(f"{self.__class__.__name__} class str ")
class C3:
def __repr__(self):
return str(f"{self.__class__.__name__} class repr")
class C4:
def __str__(self):
return str(f"{self.__class__.__name__} class str ")
def __repr__(self):
return str(f"{self.__class__.__name__} class repr")
ci1 = C1()
ci2 = C2()
ci3 = C3()
ci4 = C4()
print(ci1) #<__main__.C1 object at 0x0000024C44A80C18>
print(str(ci1)) #<__main__.C1 object at 0x0000024C44A80C18>
print(repr(ci1)) #<__main__.C1 object at 0x0000024C44A80C18>
print(ci2) #C2 class str
print(str(ci2)) #C2 class str
print(repr(ci2)) #<__main__.C2 object at 0x0000024C44AE12E8>
print(ci3) #C3 class repr
print(str(ci3)) #C3 class repr
print(repr(ci3)) #C3 class repr
print(ci4) #C4 class str
print(str(ci4)) #C4 class str
print(repr(ci4)) #C4 class repr
3
votes
Understand __str__ and __repr__ intuitively and permanently distinguish them at all.
__str__ return the string disguised body of a given object for readable of eyes__repr__ return the real flesh body of a given object (return itself) for unambiguity to identify.
See it in an example
In [30]: str(datetime.datetime.now())
Out[30]: '2017-12-07 15:41:14.002752'
Disguised in string form
As to __repr__
In [32]: datetime.datetime.now()
Out[32]: datetime.datetime(2017, 12, 7, 15, 43, 27, 297769)
Presence in real body which allows to be manipulated directly.
We can do arithmetic operation on __repr__ results conveniently.
In [33]: datetime.datetime.now()
Out[33]: datetime.datetime(2017, 12, 7, 15, 47, 9, 741521)
In [34]: datetime.datetime(2017, 12, 7, 15, 47, 9, 741521) - datetime.datetime(2
...: 017, 12, 7, 15, 43, 27, 297769)
Out[34]: datetime.timedelta(0, 222, 443752)
if apply the operation on __str__
In [35]: '2017-12-07 15:43:14.002752' - '2017-12-07 15:41:14.002752'
TypeError: unsupported operand type(s) for -: 'str' and 'str'
Returns nothing but error.
Another example.
In [36]: str('string_body')
Out[36]: 'string_body' # in string form
In [37]: repr('real_body')
Out[37]: "'real_body'" #its real body hide inside
Hope this help you build concrete grounds to explore more answers.
3
votes
__str__must return string object whereas__repr__can return any python expression.- If
__str__implementation is missing then__repr__function is used as fallback. There is no fallback if__repr__function implementation is missing. - If
__repr__function is returning String representation of the object, we can skip implementation of__str__function.
Source: https://www.journaldev.com/22460/python-str-repr-functions
0
votes
Every object inherits __repr__ from the base class that all objects created.
class Person:
pass
p=Person()
if you call repr(p) you will get this as default:
<__main__.Person object at 0x7fb2604f03a0>
But if you call str(p) you will get the same output. it is because when __str__ does not exist, Python calls __repr__
Let's implement our own __str__
class Person:
def __init__(self,name,age):
self.name=name
self.age=age
def __repr__(self):
print("__repr__ called")
return f"Person(name='{self.name}',age={self.age})"
p=Person("ali",20)
print(p) and str(p)will return
__repr__ called
Person(name='ali',age=20)
let's add __str__()
class Person:
def __init__(self, name, age):
self.name = name
self.age = age
def __repr__(self):
print('__repr__ called')
return f"Person(name='{self.name}, age=self.age')"
def __str__(self):
print('__str__ called')
return self.name
p=Person("ali",20)
if we call print(p) and str(p), it will call __str__() so it will return
__str__ called
ali
repr(p) will return
repr called "Person(name='ali, age=self.age')"
Let's omit __repr__ and just implement __str__.
class Person:
def __init__(self, name, age):
self.name = name
self.age = age
def __str__(self):
print('__str__ called')
return self.name
p=Person('ali',20)
print(p) will look for the __str__ and will return:
__str__ called
ali
NOTE= if we had __repr__ and __str__ defined, f'name is {p}' would call __str__
