What is the meaning of %r?
Question:
What’s the meaning of %r in the following statement?
print '%r' % (1)
I think I’ve heard of %s, %d, and %f but never heard of this.
Answers:
It calls repr() on the object and inserts the resulting string.
It prints the replacement as a string with repr().
See String Formatting Operations in the docs. Notice that %s and %d etc, might work differently to how you expect if you are used to the way they work in another language such as C.
In particular, %s also works well for ints and floats unless you have special formatting requirements where %d or %f will give you more control.
Background:
In Python, there are two builtin functions for turning an object into a string: str vs. repr. str is supposed to be a friendly, human readable string. repr is supposed to include detailed information about an object’s contents (sometimes, they’ll return the same thing, such as for integers). By convention, if there’s a Python expression that will eval to another object that’s ==, repr will return such an expression e.g.
>>> print repr('hi')
'hi' # notice the quotes here as opposed to...
>>> print str('hi')
hi
If returning an expression doesn’t make sense for an object, repr should return a string that’s surrounded by < and > symbols e.g. <blah>.
To answer your original question:
In addition:
You can control the way an instance of your own classes convert to strings by implementing __str__ and __repr__ methods.
class Foo:
def __init__(self, foo):
self.foo = foo
def __eq__(self, other):
"""Implements ==."""
return self.foo == other.foo
def __repr__(self):
# if you eval the return value of this function,
# you'll get another Foo instance that's == to self
return "Foo(%r)" % self.foo
Adding to the replies given above, '%r' can be useful in a scenario where you have a list with heterogeneous data type.
Let’s say, we have a list = [1, 'apple' , 2 , 'r','banana']
Obviously in this case using '%d' or '%s' would cause an error. Instead, we can use '%r' to print all these values.
%s <=> str
%r <=> repr
%r calls repr() on the object, and inserts the resulting string returned by __repr__.
The string returned by __repr__ should be unambiguous and, if possible, match the source code necessary to recreate the object being represented.
A quick example:
class Foo:
def __init__(self, foo):
self.foo = foo
def __repr__(self):
return 'Foo(%r)' % self.foo
def __str__(self):
return self.foo
test = Foo('Text')
So,
in[1]: test
Out[1]: Foo('Text')
in[2]: str(test)
Out[2]: 'Text'
The difference between %r and %s is, %r calls the repr() method and %s calls the str() method. Both of these are built-in Python functions.
The repr() method returns a printable representation of the given object.
The str() method returns the “informal” or nicely printable representation of a given object.
In simple language, what the str() method does is print the result in a way which the end user would like to see:
name = "Adam"
str(name)
Out[1]: 'Adam'
The repr() method would print or show what an object actually looks like:
name = "Adam"
repr(name)
Out[1]: "'Adam'"
%s calls the __str()__ method of the selected object and replaces itself with the return value,
%r calls the __repr()__ method of the selected object and replaces itself with the return value.
I read in “Learning Python the Hard Way”, the author said that
%r is the best for debugging, other formats are for displaying variables to users
What’s the meaning of %r in the following statement?
print '%r' % (1)
I think I’ve heard of %s, %d, and %f but never heard of this.
It calls repr() on the object and inserts the resulting string.
It prints the replacement as a string with repr().
See String Formatting Operations in the docs. Notice that %s and %d etc, might work differently to how you expect if you are used to the way they work in another language such as C.
In particular, %s also works well for ints and floats unless you have special formatting requirements where %d or %f will give you more control.
Background:
In Python, there are two builtin functions for turning an object into a string: str vs. repr. str is supposed to be a friendly, human readable string. repr is supposed to include detailed information about an object’s contents (sometimes, they’ll return the same thing, such as for integers). By convention, if there’s a Python expression that will eval to another object that’s ==, repr will return such an expression e.g.
>>> print repr('hi')
'hi' # notice the quotes here as opposed to...
>>> print str('hi')
hi
If returning an expression doesn’t make sense for an object, repr should return a string that’s surrounded by < and > symbols e.g. <blah>.
To answer your original question:
In addition:
You can control the way an instance of your own classes convert to strings by implementing __str__ and __repr__ methods.
class Foo:
def __init__(self, foo):
self.foo = foo
def __eq__(self, other):
"""Implements ==."""
return self.foo == other.foo
def __repr__(self):
# if you eval the return value of this function,
# you'll get another Foo instance that's == to self
return "Foo(%r)" % self.foo
Adding to the replies given above, '%r' can be useful in a scenario where you have a list with heterogeneous data type.
Let’s say, we have a list = [1, 'apple' , 2 , 'r','banana']
Obviously in this case using '%d' or '%s' would cause an error. Instead, we can use '%r' to print all these values.
%s <=> str
%r <=> repr
%r calls repr() on the object, and inserts the resulting string returned by __repr__.
The string returned by __repr__ should be unambiguous and, if possible, match the source code necessary to recreate the object being represented.
A quick example:
class Foo:
def __init__(self, foo):
self.foo = foo
def __repr__(self):
return 'Foo(%r)' % self.foo
def __str__(self):
return self.foo
test = Foo('Text')
So,
in[1]: test
Out[1]: Foo('Text')
in[2]: str(test)
Out[2]: 'Text'
The difference between %r and %s is, %r calls the repr() method and %s calls the str() method. Both of these are built-in Python functions.
The repr() method returns a printable representation of the given object.
The str() method returns the “informal” or nicely printable representation of a given object.
In simple language, what the str() method does is print the result in a way which the end user would like to see:
name = "Adam"
str(name)
Out[1]: 'Adam'
The repr() method would print or show what an object actually looks like:
name = "Adam"
repr(name)
Out[1]: "'Adam'"
%s calls the __str()__ method of the selected object and replaces itself with the return value,
%r calls the __repr()__ method of the selected object and replaces itself with the return value.
I read in “Learning Python the Hard Way”, the author said that
%r is the best for debugging, other formats are for displaying variables to users