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.
As far as I can gather, __str__ is just a representation of the
object. For instance:
class ServerConnectio n:
def __str__(self):
buf = "Server: " + self.name + "\n"
buf += "Sent bytes: " + str(self.sentBy tes) + "\n"
buf += "Recv bytes: " + str(self.recvBy tes) + "\n"
return buf
However, I don't understand what __repr__ should be. There's a phrase
in the documentation which makes it highly confusing for a beginner like
me: "If at all possible, this should look like a valid Python expression
that could be used to recreate an object with the same value (given an
appropriate environment).". What does that mean? Does it mean that I
should return:
def __str__(self):
buf = "self.name= " + self.name + "\n"
buf += "self.sentBytes =" + str(self.sentBy tes) + "\n"
buf += "self.recvBytes =" + str(self.recvBy tes) + "\n"
return buf
..or is there some other "valid Python expression" format which I
have yet to encounter?
15  2937 
Jan Danielsson a écrit : 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.
As far as I can gather, __str__ is just a representation of the
object.
.... yep, and this representation is built for human eyes. Don't
worry too much if it does not display every bit of information
contained in your object. Pretty printing rules ...
str(0.1)
0.1 str("it's a bad idea")
"it's a bad idea"
However, I don't understand what __repr__ should be.
It is an *exact* (if possible) description of the object's content,
nicely packaged into a string.
repr(0.1)
0.1000000000000 0001 repr("it's a bad idea")
'"it\'s a bad idea"'
There's a phrase
in the documentation which makes it highly confusing for a beginner like
me: "If at all possible, this should look like a valid Python expression
that could be used to recreate an object with the same value (given an
appropriate environment).".
It means that the equality eval(repr(x)) == x should hold.
Cheers,
SB
Errata: str(0.1)
'0.1' str("it's a bad idea")
"it's a bad idea"
repr(0.1)
' 0.1000000000000 0001' repr("it's a bad idea")
'"it\'s a bad idea"'
SB
Well, It means that eval(repr(x)) == x if at all possible.
Basically:
repr('abc') -> 'abc'
str('abc') -> abc
You'll notice that 'abc' is a valid python expression for the string,
while abc is not a valid string expression.
Andreas
On Wed, Jun 15, 2005 at 02:46:04PM +0200, 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.
As far as I can gather, __str__ is just a representation of the
object. For instance:
class ServerConnectio n:
def __str__(self):
buf = "Server: " + self.name + "\n"
buf += "Sent bytes: " + str(self.sentBy tes) + "\n"
buf += "Recv bytes: " + str(self.recvBy tes) + "\n"
return buf
However, I don't understand what __repr__ should be. There's a phrase
in the documentation which makes it highly confusing for a beginner like
me: "If at all possible, this should look like a valid Python expression
that could be used to recreate an object with the same value (given an
appropriate environment).". What does that mean? Does it mean that I
should return:
def __str__(self):
buf = "self.name= " + self.name + "\n"
buf += "self.sentBytes =" + str(self.sentBy tes) + "\n"
buf += "self.recvBytes =" + str(self.recvBy tes) + "\n"
return buf
..or is there some other "valid Python expression" format which I
have yet to encounter?
--
http://mail.python.org/mailman/listinfo/python-list
Basically __repr__ should return a string representaion of the object,
and __str__ should return a user-friendly pretty string. So maybe:
class Person:
...
def __repr__(self):
return '<Person: %s, %d, %s>' % (self.name, self.age, self.sign)
def __str__(self):
return self.name
See this for a better explanation: http://www.faqts.com/knowledge_base/...d/1331/fid/241
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.
As far as I can gather, __str__ is just a representation of the
object. [snip]
However, I don't understand what __repr__ should be.
__repr__ shouldn't be anything, if you don't have an actual need for it.
Neither should __str__.
But if you have a need for something, and you're not sure which to use,
think of it this way. If you are trying to output a representation of
the object for use in debugging, such as in a debug log file, then use
__repr__ and make it look like whatever you want, but preferably
following the <Angle brackets convention> like all the builtin stuff.
If you have a need for some representation of the data to use in your
application for non-debugging purposes, such as perhaps to display data
to a user, or to write data to an output file, then __str__ is a good
candidate, and you can make it look like whatever you need.
Just don't waste your time defining either a __repr__ or a __str__ just
because those methods exist. That won't do anyone any good.
-Peter
In article <42********@gri seus.its.uu.se> ,
Jan Danielsson <ja************ @gmail.com> 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.
As far as I can gather, __str__ is just a representation of the
object.
No, it is not. It is a conversion to string. The result has
no trace of the original object, per se -- in principle, many
different kinds of object could yield the same result. Suppose
str(x) -> "42". Is x an int? a string? an XMLParseResult?
AnswerToLifeThe UniverseAndEver ything? You don't want to know,
that's why you use str().
The "friendly" idea is vacuous if you look hard enough at it,
and won't really help.
However, I don't understand what __repr__ should be. There's a phrase
in the documentation which makes it highly confusing for a beginner like
me: "If at all possible, this should look like a valid Python expression
that could be used to recreate an object with the same value (given an
appropriate environment).". What does that mean?
It's kind of a bad idea that the Python world isn't ready to
let go of yet.
repr() is really the representation of the object. If you
look at the result of repr(x), you should indeed see some
clue to the original object. I guess this is what gives
people the idea that str() is friendly, because as a rule
users are not interested in the original object -- but
there are exceptions, for example you may find the quotes
useful around a string, depending on the situation. The
object information also leads to the marshalling idea, but
of course this isn't (and can't be) implemented for many
objects so we can't expect this to work reliably with
random objects. The price we pay for the notion is mostly
in the "fix" for float repr, which now displays the float
in all its imprecise glory and routinely confounds people
who don't understand what that's about.
Donn Cave, do**@u.washingt on.edu
On Wednesday 15 June 2005 08:06 am, Sébastien Boisgérault wrote: Jan Danielsson a écrit : However, I don't understand what __repr__ should be.
It is an *exact* (if possible) description of the object's content,
nicely packaged into a string.
However, this is often not the case. Frequently __repr__ will
return something like this: import re
r = re.compile('\w+ \d*')
r
<_sre.SRE_Patte rn object at 0x401a89b0> str(r)
'<_sre.SRE_Patt ern object at 0x401a89b0>' repr(r)
'<_sre.SRE_Patt ern object at 0x401a89b0>'
So don't obsess over it. For many objects it isn't worth
the trouble, and for others, str() and repr() are sensibly the
same thing, but for some, the distinction is useful. That's
all.
--
Terry Hancock ( hancock at anansispacework s.com )
Anansi Spaceworks http://www.anansispaceworks.com
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
"Jan Danielsson" <ja************ @gmail.com> wrote in message
news:42******** @griseus.its.uu .se... 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.
As far as I can gather, __str__ is just a representation of the
object. For instance:
class ServerConnectio n:
def __str__(self):
buf = "Server: " + self.name + "\n"
buf += "Sent bytes: " + str(self.sentBy tes) + "\n"
buf += "Recv bytes: " + str(self.recvBy tes) + "\n"
return buf
However, I don't understand what __repr__ should be. There's a phrase
in the documentation which makes it highly confusing for a beginner like
me: "If at all possible, this should look like a valid Python expression
that could be used to recreate an object with the same value (given an
appropriate environment).". What does that mean? Does it mean that I
should return:
def __str__(self):
buf = "self.name= " + self.name + "\n"
buf += "self.sentBytes =" + str(self.sentBy tes) + "\n"
buf += "self.recvBytes =" + str(self.recvBy tes) + "\n"
return buf
..or is there some other "valid Python expression" format which I
have yet to encounter?
str() should be something that's meaningful to a human being when
it's printed or otherwise rendered. repr() should be something that
can round trip: that is, if you feed it into eval() it should reproduce
the object.
You can't always achieve either one, especially with very
complex objects, but that's the goal.
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