What does !r do in str() and repr()?
In order to format something in a string, a string representation of that something must first be created. "convert the value" is basically talking about how the string representation is to be constructed. In python, there are two fairly natural choices to get a string representation of something ... str
and repr
. str
is generally a little more human friendly, repr
is generally more precise. Perhaps the official documentation is the best place to go looking for the difference:
object.__repr__(self)
Called by the
repr()
built-in function to compute the “official” string representation of an object. 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). If this is not possible, a string of the form<...some useful description...>
should be returned. The return value must be a string object. If a class defines__repr__()
but not__str__()
, then__repr__()
is also used when an “informal” string representation of instances of that class is required.This is typically used for debugging, so it is important that the representation is information-rich and unambiguous.
object.__str__(self)
Called by str(object) and the built-in functions format() and print() to compute the “informal” or nicely printable string representation of an object. The return value must be a string object.
This method differs from
object.__repr__()
in that there is no expectation that__str__()
return a valid Python expression: a more convenient or concise representation can be used.The default implementation defined by the built-in type object calls
object.__repr__()
.
In str.format
, !s
chooses to use str
to format the object whereas !r
chooses repr
to format the value.
The difference can easily be seen with strings (as repr
for a string will include outer quotes).:
>>> 'foo {}'.format('bar')
'foo bar'
>>> 'foo {!r}'.format('bar')
"foo 'bar'"
What the difference between these two methods really depends critically on the objects being formatted. For many objects (e.g. those that don't override the __str__
method), there will be no difference in the formatted output.
nalzok
The best thing about being a statistician is that you get to play in everyone's backyard.
Updated on July 09, 2022Comments
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nalzok almost 2 years
According to the Python 2.7.12 documentation:
!s
(applystr()
) and!r
(applyrepr()
) can be used to convert the value before it is formatted.>>> import math >>> print 'The value of PI is approximately {}.'.format(math.pi) The value of PI is approximately 3.14159265359. >>> print 'The value of PI is approximately {!r}.'.format(math.pi) The value of PI is approximately 3.141592653589793.
Interestingly, the converted value is the output of
repr()
, rather thanstr()
.>>> str(math.pi) '3.14159265359' >>> repr(math.pi) '3.141592653589793'
So what does "convert the value" mean here? Making it less human-readable?
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Admin over 2 yearsYour answer could be improved with additional supporting information. Please edit to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center.
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Gino Mempin over 2 yearsThis doesn't seem to answer the question of
!r
and!s
instr.format
. Also, please edit to provide details of what you mean by "informal". -
mehmet over 2 yearsExplicit is better than implicit, use
repr
instead of!r
.