str() vs repr() functions in python 2.7.5 [duplicate]

Question

what is the difference between str() and repr() functions in python 2.7.5?

Explanation on python.org:

The str() function is meant to return representations of values which are fairly human-readable, while repr() is meant to generate representations which can be read by the interpreter (or will force a SyntaxError if there is no equivalent syntax)

But it wasn't clear for me.

some examples:

>>> s = 'Hello, world.'
>>> str(s)
'Hello, world.'
>>> repr(s)
"'Hello, world.'"      # repr is giving an extra double quotes
>>> str(1.0/7.0)
'0.142857142857'
>>> repr(1.0/7.0)
'0.14285714285714285'  # repr is giving value with more precision

so I want to know the following

1. When should I use str() and when should I use repr()?
2. In which cases I can use either of them?
3. What can str() do which repr() can't?
4. What can repr() do which str() can't?

Phew

repr should return the Python "object representation" that evaluates to such object, as applicable. This is why the string is quoted when using repr: so eval(repr(someStr)) == someStr should is true (it would also have nicely escaped unprintable and control characters). It (repr and reprlib) is useful for debugging and exploring objects, but should generally not be used for end-user output. – Kero

check this satyajit.ranjeev.in/2012/03/14/python-repr-str.html and #1437203 – Philosophical

Note that when you just type str(s) in the interpreter, it's printing out repr(str(s)), because the interpreter displays the repr of any expression you type. print str(s) and print repr(s) and you might find it more enlightening. – Nook

Note that in your numbers example, repr() may be giving you a higher precision, but not necessarily a higher accuracy. It does show more digits, but because of typical floating point limitations, the value shown by str() may be better. Compare repr(sum(0.1 for i in range(9))) and str(sum(0.1 for i in range(9))). On the other hand, str() will hide the inherent inaccuracy from you, which confuses people who wonder why sum(0.1 for i in range(9)) == 0.9 returns False... – Hiphuggers

@hcwhsa I didn't understand why my question is duplicate.I saw the link which you gave.But there the explanation was given using classes,self. I am an absolute beginner of python. I didn't even understand it.And my doubt is on functions and i clearly explained it.So can you please reconsider my question ? – Phew

I have to agree with @vnvmabhinav. His question seeks answer as a Python language beginner, a noob who has just started reading The Python Tutorial. The link that you have provided explains more about the implementations and intentions of str and repr in class definitions. I .. just hate this. :( Well, its 3 and 1/2 years later, so I shouldn't complain. – Buonomo

Answer 1

When should i use str() and when should i use repr() ?

Almost always use str() when creating output for end users.

repr() is mainly useful for debugging and exploring. For example, if you suspect a string has non printing characters in it, or a float has a small rounding error, repr() will show you; str() may not.

repr() can also be useful for generating literals to paste into your source code. It can also be used for persistence (with ast.literal_eval or eval), but this is rarely a good idea--if you want editable persisted values, something like JSON or YAML is much better, and if you don't plan to edit them, use pickle.

2.In which cases i can use either of them ?

Well, you can use them almost anywhere. You shouldn't generally use them except as described above.

3.What can str() do which repr() can't ?

Give you output fit for end-user consumption--not always (e.g., str(['spam', 'eggs']) isn't likely to be anything you want to put in a GUI), but more often than repr().

4.What can repr() do which str() can't

Give you output that's useful for debugging--again, not always (the default for instances of user-created classes is rarely helpful), but whenever possible.

And sometimes give you output that's a valid Python literal or other expression--but you rarely want to rely on that except for interactive exploration.

Nook

You said: "repr can also be useful for for generating literals to paste into your source code." Another way to do this would be to use what are called Raw Strings. – Buonomo

@RedJohn Raw strings literals don't do anything like the same thing. There may be a tiny bit of overlap (if you don't know how to escape a string, you should probably use a raw string, but you could instead use repr to show you how to escape it), but that's it. – Nook

Agreed. I was just talking about that one similarity. I am not claiming any more. I, for one, make mistakes when writing file names in source code; especially in Windows where there are backslashes in file names. So, there I use raw strings instead of copy-pasting the proper escape-sequenced string using repr() in the interpreter. – Buonomo