Re: Python Question About Format Specifiers
- From: David Tseng <davidct1209@xxxxxxxxx>
- To: programmingblind@xxxxxxxxxxxxx
- Date: Tue, 7 Jun 2011 11:15:57 -0700
>>> class foo:
... def __str__(self):
... return "bar"
...
>>> f = foo()
>>> f
<__main__.foo instance at 0x1004d2638>
>>> str(f)
'bar'
>>> repr(f)
'<__main__.foo instance at 0x1004d2638>'
>>>
class foo overrides the base implementation of __str__; hence, the
result is the output above when "casting"/converting to str (not a
python junky, so Chris can fill you in on the proper terminology). As
a result, as Chris pointed out, if you did something exceptional in
__str__ (like dividing by 0), you would not even have the above output
when getting the readable string via str().
On 6/7/11, Homme, James <james.homme@xxxxxxxxxxxx> wrote:
> Hi Chris,
> This is my understanding of what you said. If you really want to see what
> something looks like, use %r. If you want to see how it is represented by
> some object, use %s. Is that anywhere close?
>
> Thanks.
>
> Jim
>
> -----Original Message-----
> From: programmingblind-bounce@xxxxxxxxxxxxx
> [mailto:programmingblind-bounce@xxxxxxxxxxxxx] On Behalf Of Q
> Sent: Monday, June 06, 2011 11:51 PM
> To: programmingblind@xxxxxxxxxxxxx
> Subject: Re: Python Question About Format Specifiers
>
> %r vs. %s...
> Use cases:
> Use standard format characters (%s, %d, %.2f) when formatting output you
> can trust.
> Use %r when you need to output something you cannot trust, for instance
> if you're logging a failed condition and don't actually know current
> state of some object. For instance:
>
> try:
> do_something(some_val)
> except SomeStrangeError:
> logging.exception("Failed to do something. Value of some_val was %r"
> % some_val
>
> Because an exception was thrown, and at this point we don't know why, we
> want to communicate the state of the program as well as we can to the
> log. This means printing out the raw state of the referenced some_val
> using %r. Then we don't have to worry about the format characters
> potentially munging our some_val into a different format for
> presentation and potentially hiding something.
> Also, since __str__, the method which is called when you go
> str(some_obj), which is what's going on under the covers when you use %s
> in your statements, might have side effects. Consider the case of an
> object which is initialized with a name, and whose __str__ method prints
> a representation of the object which includes that name.
> Then, imagine you've got code logging with that object, instantiate one,
> forget to give it a name, so the __str__ method raises an exception. So
> if then you try to log an exception with a string representation of the
> object, that also raises an exception and your log doesn't get written.
> This would obviously be poor design, as an object should complain about
> not getting a name by default, but it is for illustrative purposes.
> I hope this demystifies things a bit,
> Q
>
>
> On 6/6/2011 3:52 PM, Homme, James wrote:
>> Hi Dave,
>> I read in the Python Standard Library documentation about %r and %s, and
>> one said that it uses repr, and the other said that it uses str. Next
>> time, I'll follow the documentation path the whole way and actually go and
>> read about the functions. Thanks for being so nice about answering this.
>>
>> Jim
>>
>> -----Original Message-----
>> From: programmingblind-bounce@xxxxxxxxxxxxx
>> [mailto:programmingblind-bounce@xxxxxxxxxxxxx] On Behalf Of David Tseng
>> Sent: Monday, June 06, 2011 3:36 PM
>> To: programmingblind@xxxxxxxxxxxxx
>> Subject: Re: Python Question About Format Specifiers
>>
>> http://stackoverflow.com/questions/1436703/difference-between-str-and-repr-in-python
>>
>> In other words, it's the difference between repr() and str().
>>
>> On 6/6/11, Homme, James<james.homme@xxxxxxxxxxxx> wrote:
>>> Hi,
>>> My goal in this program is to properly explain above lines what they are
>>> doing. I'm unsure what goes on with the last line. I see what it's doing
>>> when I run the program, but I don't know why it's doing it.
>>>
>>> # Get some input from the user and work with it.
>>> #
>>> # Print and use a comma to make the input go to the same line as the
>>> question.
>>> print "How old are you?",
>>> # Get input and put it into age.
>>> age = raw_input()
>>> # Now, give the prompt and get the input all at once.
>>> # Note that now we have to put a space at the end of the quoted string.
>>> height = raw_input("How tall are you? ")
>>> # Now do it again.
>>> weight = raw_input("How much do you weigh? ")
>>> # Now print out the variables with raw string format specifiers.
>>> # This turns them into single quoted strings properly formatted.
>>> print "So, you're %r old, %r tall and %r heavy." % ( age, height, weight)
>>> # I'm unsure how changing this to %s does what it does.
>>> print "So number 2, you're %s old, %s tall and %s heavy." % ( age,
>>> height,
>>> weight)
>>>
>>> Thanks.
>>>
>>> Jim
>>> Jim Homme,
>>> Usability Services,
>>> Phone: 412-544-1810.
>>>
>>>
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