Help with format characters.

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December 19th, 2012, 02:32 PM

whitewinter

Registered User

Join Date: Dec 2012

Posts: 2

Time spent in forums: 8 m 47 sec
Reputation Power: 0

Help with format characters.

I am new to python and have a little question about format characters.

I did a few tests with them(I think the best way to learn is try it out) and I can't find the difference between %r and %s. It works with a floating point number, a number and strings. While all the others don't work with at least one of them. I already find out that %s converts the variable into a string with str() and %r with repr(). I also thought that %r is used for debugging.

A little example:

Code:

string = "This is just a string."
number = 3
floating_point = 3.0


# all the variables with %r

print "This is what I'm printing: %r" % string
print "This is what I'm printing: %r" % number
print "This is what I'm printing: %r" % floating_point


# all the variables with %s

print "This is what I'm printing: %s" % string
print "This is what I'm printing: %s" % number
print "This is what I'm printing: %s" % floating_point


# all the variables with %c(I know it will fail with 2 of them # 
# because it is used for things with 1 character).

print "This is what I'm printing: %c" % string
print "This is what I'm printing: %c" % number
print "This is what I'm printing: %c" % floating_point

Output:

Code:

This is what I'm printing: 'This is just a string.'
This is what I'm printing: 3
This is what I'm printing: 3.0
This is what I'm printing: This is just a string.
This is what I'm printing: 3
This is what I'm printing: 3.0
Traceback (most recent call last):
  File "3.py", line 23, in <module>
    print "This is what I'm printing: %c" % string
TypeError: %c requires int or char

December 19th, 2012, 03:16 PM

Nyktos

Contributing User

Join Date: Dec 2012

Posts: 76

Time spent in forums: 1 Day 3 h 26 m 10 sec
Reputation Power: 2

Well...you've already found the difference then. %s uses str() and %r uses repr(). It just so happens that these are the same for most built-in types. If you define a class for which they are not the same...

Code:

class Foo:
    def __str__(self):
        return "blah blah"
    
    def __repr__(self):
        return "do dee do"

...you will see a difference:

Code:

>>> foo = Foo()
>>> "using s: %s" % foo
'using s: blah blah'
>>> "using r: %r" % foo
'using r: do dee do'

Normally __repr__ is meant to return something that could be entered into the interpreter to recreate the object while __str__ should return something "printable" (as str() is called by print()).

December 20th, 2012, 04:31 AM

whitewinter

Registered User

Join Date: Dec 2012

Posts: 2

Time spent in forums: 8 m 47 sec
Reputation Power: 0

Thank you!

Help with format characters.

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