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Situation: I'm new to python and currently trying to learn the ropes, I've attempted creating a linked list class to assist in getting a better understanding of the language and its structures. I know that the __repr__ function is basically supposed to return the same thing as __str__ but I'm unsure on what the actual difference is.

Here's my class so far: 

class LinkedList: class Node: def __init__(self, val, prior=None, next=None): self.val = val self.prior = prior self.next = next def __init__(self): self.head = LinkedList.Node(None) self.head.prior = self.head.next = self.head self.length = 0 def __str__(self): """Implements `str(self)`. Returns '[]' if the list is empty, else returns `str(x)` for all values `x` in this list, separated by commas and enclosed by square brackets. E.g., for a list containing values 1, 2 and 3, returns '[1, 2, 3]'.""" if len(self)==0: return '[]' else: return '[' + ', '.join(str(x) for x in self) + ']' def __repr__(self): """Supports REPL inspection. (Same behavior as `str`.)""" return '[' + ', '.join(str(x) for x in self) + ']'

When I test this code against the below code, I'll get an error basically saying the blank string '[]' isn't actually being returned when using the repr function. How could I edit this methods body to fix this issue? I've also tried return str(self) and I'm not sure why that won't work either.

from unittest import TestCase tc = TestCase() lst = LinkedList() tc.assertEqual('[]', str(lst)) tc.assertEqual('[]', repr(lst)) lst.append(1) tc.assertEqual('[1]', str(lst)) tc.assertEqual('[1]', repr(lst))
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    "I know that the __repr__ function is basically supposed to return the same thing as __str__" - I'm not sure how you know that, but that's not right, see the docs on __str__ and __repr__. Commented Jun 12, 2016 at 21:13
  • The code you've shown can't be run as written (you've left out the __len__ and __iter__ methods, at least), which makes troubleshooting it very difficult. While we appreciate a concise question, we need a minimal reproducible example that actually works! Commented Jun 13, 2016 at 0:34
  • This has all the answers: Difference between str and repr? Commented Dec 4, 2020 at 9:38

1 Answer 1

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The __repr__ function returns a string representation of a Python object that may be evaluated by the Python interpreter to instantiate another instance of the object. So if you had a list:

x = ['foo', 'bar']

Its __repr__ string would be:

x_str = repr(x) print(x_str) >>>> "['foo', 'bar']"

And you could do:

x2 = eval(x_str) print(type(x2)) >>>> <class 'list'>

It's a way to get a string representation of a Python object that can be converted back into a new instance of said object.

Basically the difference between __str__ and __repr__ is that the former returns a string representation of the object meant to be read by a person and the latter returns a string representation of the object meant to be parsed by the Python interpreter. Be very careful with this!

In your example code, it appears that __str__ and __repr__ return the same string representation. That's fine. However, if you wanted, you could make your __str__ return some prettier formatted version (for instance with carriage returns and no brackets), but __repr__ should always return a string that could be parsed by the Python interpreter to reconstruct the object.

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This is a great way to think about repr! I was wondering how I should structure my output and this answers it great. However, it should be noted that repr and eval should not be used as a serializer because there are instances where this is broken. But this is a great standard for creating repr's.