What is the difference between __str__
and __repr__
in Python?
28 Answers
Alex Martelli 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__)
Or in new f-string formatting:
return f"{self.__class__!s}({self.__dict__!r})"
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(lst) + "]")
(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.
-
389Definitely disagree with your opinion that debugging isn't the way to go. For development use a debugger (and/or logging), for production use logging. With a debugger you have a view of everything that went wrong when the problem occurred. You can see the full picture. Unless you are logging EVERYTHING you can't get that. Plus if you are logging everything you're going have to wade through tons of data to get at what you want.– SamuelCommented Feb 21, 2015 at 19:06
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48Great answer (except the bit about not using debuggers). I'd just like to add a link to this other Q&A about str vs unicode in Python 3 which could be relevant to the discussion for people who have made the switch. Commented Mar 23, 2015 at 21:04
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38on debugger vs no debugger: don't get such entrenched opinions. In some applications debugging is not realistic, typically when real-time is involved, or when your code only executes remotely on a platform with little access or no console. In most other cases it will be much quicker to stop at an exception to investigate, or to set a breakpoint, because you don't have to go through thousands of lines of logging (which will clutter your disk and slow down the application). Finally, it's not always possible to log, for example on embedded devices, there debugger is your friend too.– RedGlyphCommented Aug 2, 2019 at 11:42
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12About debuggging vs logging, they are both useful. If a bug is reproducible, debugging is more simple. If the bug is randomic, logging is essential. Commented Jan 15, 2020 at 15:16
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15A small update for recent versions of Python 3: you can use f-strings and still invoke the
__repr__
of data you embed, by adding!r
: you can replace"MyClass(this=%r,that=%r)" % (self.this,self.that)
byf"MyClass(this={self.this!r},that={self.that!r})"
. Otherwise, thanks for this great post!– joanisCommented May 12, 2021 at 18:36
My rule of thumb: __repr__
is for developers, __str__
is for customers.
-
26This is true because for obj = uuid.uuid1(), obj.__str__() is "2d7fc7f0-7706-11e9-94ae-0242ac110002" and obj.__repr__() is "UUID('2d7fc7f0-7706-11e9-94ae-0242ac110002')". Developers need (value + origin) whereas customers need a value and they don't care how they got it! Commented May 15, 2019 at 11:45
-
16Here customer may not necessarily mean end-user. It's the client or user of the object. So if its an SDK then the SDK developers will use
__str__
so normal developers have readable object. On the other hand,__repr__
is for the SDK developers themselves. Commented Jan 9, 2020 at 13:16 -
2@NarenYellavula if you're exposing a UUID to a customer you're probably doing something wrong. Commented Dec 18, 2021 at 19:37
-
@AbdessabourMtk they're overly complex, and there's no protection against typing them wrong. Maybe in certain contexts like as part of a QR code they would be OK. Commented Apr 13, 2022 at 14:02
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1@Mark Customers can be technical too, like for example GParted exposes UUIDs for partitions (screenshot). If you want an example that works for non-technical customers:
d = datetime.date.today()
str:2023-05-20
repr:datetime.date(2023, 5, 20)
– wjandreaCommented May 20, 2023 at 20:06
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 eval
!
-
55In my unit tests I always check that
eval(repr(foo))
evaluates to an object equal tofoo
. You're right that it won't work outside of my test cases since I don't know how the module is imported, but this at least ensures that it works in some predictable context. I think this a good way of evaluating if the result of__repr__
is explicit enough. Doing this in a unit test also helps ensure that__repr__
follows changes to the class. Commented Nov 15, 2011 at 19:58 -
8I always try to make sure that either
eval(repr(spam)) == spam
(at least in the right context), oreval(repr(spam))
raises aSyntaxError
. That way you avoid confusion. (And that's almost true for the builtins and most of the stdlib, except for, e.g., recursive lists, wherea=[]; a.append(a); print(eval(repr(a)))
gives you[[Ellipses]]
…) Of course I don't do that to actually useeval(repr(spam))
, except as a sanity check in unit tests… but I do sometimes copy and pasterepr(spam)
into an interactive session.– abarnertCommented Sep 20, 2014 at 5:34 -
1Why would not containers (lists, tuples) use
__str__
for each element instead of__repr__
? Seems plain wrong to me, as I implemented a readable__str__
in my object and when it is part of a list I see the uglier__repr__
instead.– SuperGeoCommented Jan 26, 2018 at 14:15 -
1@SuperGeo Other answers cover this: container
str
use elementrepr
because[1, 2, 3]
!=["1", "2, 3"]
.– mtraceurCommented Dec 6, 2019 at 8:15 -
3
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,''
.
-
3What is the meaning of printable string here? Can you explain it please?– VicrobotCommented Aug 19, 2018 at 5:44
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3building upon the above example by "bitoffdev" and @deadly we can see how str is for the end user because it only gives us a readable string where as repr is fro developers because it gives us the value as well as the type. If you are looking for interview answers then it would be perfect.– PSK0007Commented Jul 13, 2020 at 18:40
__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
-
4__repr__() : used to create "constructor-like expression" in string, so that eval() can re-construct an object back from this string representation __str__() : used to create string containing a printable representation of an object Commented Dec 31, 2021 at 7:16
-
This answer would a bit clearer if the commands in the REPL were
print(repr(s))
andprint(str(s))
. Without theprint(...)
, the output includes extra quotes and escaping that aren't actually part of the content of those strings. Commented Feb 15, 2024 at 2:39
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(
type(self).__module__, type(self).__qualname__, 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.
-
1Shouldn't it be something along the lines of
type(obj).__qualname__
? Commented Dec 1, 2018 at 1:10 -
@SolomonUcko yes in Python 3, that would seem to be the case - I've been hunting down the source code where this is implemented and I'll update my answer with that information when I get it together.– Aaron Hall ♦Commented Dec 1, 2018 at 20:50
-
This answer will be more helpful for beginners. Nice explanation!! Commented Feb 6, 2022 at 6:48
-
I have changed
self.__module__
totype(self).__module__
(since for exemple3
has no__module__
) andtype(self).__name__
totype(self).__qualname__
(since for instance withclass A: class B: pass
that is whatrepr(A.B())
returns). Commented Mar 31, 2022 at 17:06
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'
-
-
10It's kind of misleading to refer to
repr
as reproduce. It is better to think of it as represent. Commented Jun 19, 2019 at 7:48 -
1@NelsonGon he is not totally wrong, docs say - "this should look like a valid Python expression that could be used to recreate an object with the same value (given an appropriate environment). If this is not possible, a string of the form <...some useful description...> should be returned." so it makes sense to in a way think of this as reproducing the object, as many have pointed out the use of
eval
Commented Oct 29, 2023 at 17:25
Apart from all the answers given, I would like to add few points :-
__repr__()
is invoked when you simply write object's name on interactive python console and press enter.__str__()
is invoked when you use object with print statement.In case, if
__str__
is missing, then print and any function usingstr()
invokes__repr__()
of object.__str__()
of containers, when invoked will execute__repr__()
method of its contained elements.str()
called within__str__()
could potentially recurse without a base case, and error on maximum recursion depth.__repr__()
can callrepr()
which will attempt to avoid infinite recursion automatically, replacing an already represented object with...
.
-
"
repr()
which will attempt to avoid infinite recursion automatically, replacing an already represented object with...
" — Where did you get that from? It doesn't do that. Maybe you're thinking ofreprlib.recursive_repr
– wjandreaCommented Oct 15, 2023 at 6:46 -
@wjandrea I see the
...
behavior in an interactive python 3.11 session:a = [0,1,2]; a.append(a); repr(a)
outputs'[0, 1, 2, [...]]'
Commented Jun 11, 2024 at 0:53 -
@DonHatch Yeah, builtin types have that functionality, but if you implement your own class with its own
__repr__
, it doesn't happen automatically. That's whatreprlib.recursive_repr
is for.– wjandreaCommented Jun 11, 2024 at 1:28
TL;DR:
LONG
When it comes down to the nitty-gritty, this question is analogous to asking 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 please 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 expression, an object 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 is 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
. Similarly, 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
So this tells you the Python shell just eval()
s what you give it. Now, let's repr()
both expressions and see what we get. More specifically, take its output and dump those out in the interpreter (there's a point to this which we'll address afterwards):
>>> 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 useprint
, even if they 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).
-
Don't post pictures of text. Stack Exchange supports table formatting now if you want to use that.– wjandreaCommented Oct 15, 2023 at 5:45
-
1"
repr()
&__repr__()
return a string representation of an object that is a valid Python expression" — Not always. Yes it's a goal, but some objects can't be meaningfully represented, likeobject
for example:object()
→<object object at 0x7f4aa8b38f50>
. I know other answers cover this and you're probably aware of it, but I found it strange you didn't mention it anywhere in this long explanation.– wjandreaCommented Oct 15, 2023 at 6:06 -
1Yep, I didn't know table-formatting was available... will convert that soon. As far as unrepresentable objects, yeah,
repr()
shows the default "object string" and itsid()
. I probably should've mentioned it and specify it's the default if you don't implement__str__()
or__repr()__
. I'll see if I can weave it into the response when I fix the table. Thx for the useful comments!– wescpyCommented May 9, 2024 at 3:27
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)
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."
-
4
__repr__
is by no means required to return a vaild Python expression. Commented Oct 20, 2017 at 3:23
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.
-
26I think this misses the point. The use of
eval(repr(obj))
is a sanity test and a rule of thumb - if this recreates the original object correctly then you have a decent__repr__
implementation. It's not intended that you actually serialize objects this way.– jwgCommented Jun 6, 2014 at 13:56 -
10
eval
is not inherently dangerous. Is not more dangerous thanunlink
,open
, or writing to files. Should we stop writing to files because perhaps a malicious attack could use an arbitrary file path to put content inside? Everything is dangerous if dumbly used by dumb people. Idiocy is dangerous. Dunning-Kruger effects are dangerous.eval
is just a function. Commented Mar 1, 2016 at 15:12
You can get some insight from this code:
class Foo:
def __repr__(self):
return "repr"
def __str__(self):
return "str"
foo = Foo()
foo # repr
print(foo) # str
str
- Creates a new string object from the given object.
repr
- Returns the canonical string representation of the object.
The differences:
str():
- makes object readable
- generates output for end-user
repr():
- needs code that reproduces object
- generates output for developer
__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
From the book Fluent Python:
A basic requirement for a Python object is to provide usable string representations of itself, one used for debugging and logging, another for presentation to end users. That is why the special methods
__repr__
and__str__
exist in the data model.
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.
>>> 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.
The difference between __str__
and __repr__
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))
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.
-
It uses
__repr__
when it (__str__
) is not defined! So, you are wrong.– jitenCommented Dec 21, 2018 at 13:48
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'
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.
__repr__
is used everywhere, except by print
and str
methods (when a __str__
is defined !)
__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
-
"
__repr__
can return any python expression" — Where did you get that from?__repr__
also needs to return a string, plus expressions are elements of syntax, not objects, so they can't be "returned".– wjandreaCommented Oct 15, 2023 at 6:20 -
"There is no fallback if
__repr__
function implementation is missing." — There is a fallback:object.__repr__
– wjandreaCommented Oct 15, 2023 at 6:22
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__
Programmers with prior experience in languages with a
toString
method tend to implement__str__
and not__repr__
. If you only implement one of these special methods in Python, choose__repr__
.
From Fluent Python book, by Ramalho, Luciano.
repr() used when we debug or log.It is used for developers to understand code.
one the other hand str() user for non developer like(QA) or user.
class Customer:
def __init__(self,name):
self.name = name
def __repr__(self):
return "Customer('{}')".format(self.name)
def __str__(self):
return f"cunstomer name is {self.name}"
cus_1 = Customer("Thusi")
print(repr(cus_1)) #print(cus_1.__repr__())
print(str(cus_1)) #print(cus_1.__str__())
Basically __str__
or str() is used for creating output that is human-readable are must be for end-users.
On the other hand, repr() or __repr__
mainly returns canonical string representation of objects which serve the purpose of debugging and development helps the programmers.