When to use %r instead of %s in Python?
Question:
On Learn Python the Hard Way page 21, I see this code example:
x = "There are %d types of people." % 10
...
print "I said: %r." % x
Why is %r used here instead of %s? When would you use %r, and when would you use %s?
Answers:
The %s specifier converts the object using str(), and %r converts it using repr().
For some objects such as integers, they yield the same result, but repr() is special in that (for types where this is possible) it conventionally returns a result that is valid Python syntax, which could be used to unambiguously recreate the object it represents.
Here’s an example, using a date:
>>> import datetime
>>> d = datetime.date.today()
>>> str(d)
'2011-05-14'
>>> repr(d)
'datetime.date(2011, 5, 14)'
Types for which repr() doesn’t produce Python syntax include those that point to external resources such as a file, which you can’t guarantee to recreate in a different context.
%r shows with quotes:
It will be like:
I said: 'There are 10 types of people.'.
If you had used %s it would have been:
I said: There are 10 types of people..
This is a version of Ben James’s answer, above:
>>> import datetime
>>> x = datetime.date.today()
>>> print x
2013-01-11
>>>
>>>
>>> print "Today's date is %s ..." % x
Today's date is 2013-01-11 ...
>>>
>>> print "Today's date is %r ..." % x
Today's date is datetime.date(2013, 1, 11) ...
>>>
When I ran this, it helped me see the usefulness of %r.
Use the %r for debugging, since it displays the “raw” data of the variable,
but the others are used for displaying to users.
That’s how %r formatting works; it prints it the way you wrote it (or close to it). It’s the “raw” format for debugging. Here n used to display to users doesn’t work. %r shows the representation if the raw data of the variable.
months = "nJannFebnMarnAprnMaynJunnJulnAug"
print "Here are the months: %r" % months
Output:
Here are the months: 'nJannFebnMarnAprnMaynJunnJulnAug'
Check this example from Learn Python the Hard Way.
On Learn Python the Hard Way page 21, I see this code example:
x = "There are %d types of people." % 10
...
print "I said: %r." % x
Why is %r used here instead of %s? When would you use %r, and when would you use %s?
The %s specifier converts the object using str(), and %r converts it using repr().
For some objects such as integers, they yield the same result, but repr() is special in that (for types where this is possible) it conventionally returns a result that is valid Python syntax, which could be used to unambiguously recreate the object it represents.
Here’s an example, using a date:
>>> import datetime
>>> d = datetime.date.today()
>>> str(d)
'2011-05-14'
>>> repr(d)
'datetime.date(2011, 5, 14)'
Types for which repr() doesn’t produce Python syntax include those that point to external resources such as a file, which you can’t guarantee to recreate in a different context.
%r shows with quotes:
It will be like:
I said: 'There are 10 types of people.'.
If you had used %s it would have been:
I said: There are 10 types of people..
This is a version of Ben James’s answer, above:
>>> import datetime
>>> x = datetime.date.today()
>>> print x
2013-01-11
>>>
>>>
>>> print "Today's date is %s ..." % x
Today's date is 2013-01-11 ...
>>>
>>> print "Today's date is %r ..." % x
Today's date is datetime.date(2013, 1, 11) ...
>>>
When I ran this, it helped me see the usefulness of %r.
Use the %r for debugging, since it displays the “raw” data of the variable,
but the others are used for displaying to users.
That’s how %r formatting works; it prints it the way you wrote it (or close to it). It’s the “raw” format for debugging. Here n used to display to users doesn’t work. %r shows the representation if the raw data of the variable.
months = "nJannFebnMarnAprnMaynJunnJulnAug"
print "Here are the months: %r" % months
Output:
Here are the months: 'nJannFebnMarnAprnMaynJunnJulnAug'
Check this example from Learn Python the Hard Way.