Python's str() vs. repr()

April 10, 2015

Ever wondered what happens when you call Python’s built-in str(X), with X being any object you want? The return value of this function depends on the two magic methods __str__ being the first choice and __repr__ as a fallback. But what’s the difference between them? When having a look at the docs

>>> help(str)
'Create a new string object from the given object.'
>>> help(repr)
'Return the canonical string representation of the object.'

they seem to be fairly similar. Let’s see them in action:

>>> str(123)
'123'
>>> repr(123)
'123'

Alright no difference for now.

>>> str('Python')
'Python'
>>> repr('Python')
"'Python'"

A second pair of quotes around our string. Why?
With the return value of repr() it should be possible to recreate our object using eval(). This function takes a string and evaluates it’s content as Python code. In our case passing “‘Python’“ to it works, whereas ‘Python’ leads to an error cause it’s interpreted as the variable Python which is of course undefined. Let’s move on…

>>> import datetime
>>> now = datetime.datetime.now() 
>>> str(now)
'2015-04-04 20:51:31.766862'
>>> repr(now)
'datetime.datetime(2015, 4, 4, 20, 51, 31, 766862)'

This is some significant difference. While str(now) computes a string containing the value of now, repr(now) again returns the Python code needed to rebuild our now object.</br>

The following clues might help you to decide when to use which:

str() repr()
- make object readable - need code that reproduces object
- generate output for end user - generate output for developer


These points should also be considered when writing __str__ or __repr__ for your classes.


If you have questions or want to give feedback feel free to contact me.