Why python designed as str(None) return 'None' instead of an empty string?

Question

In some other languages I knows, the intuitive result of a null to string conversion should be an empty string. Why Python is designed to make 'None' be sort of special string? And this can lead to extra work when checking a return value from a function

result = foo() # foo will return None if failure 
if result is not None and len(str(result)) > 0:
    # ... deal with result 
    pass

if str(None) returns empty string, the code could be shorter:

if len(str(result)) > 0:
    # ... deal with result 
    pass

Looks like Python is trying to be verbose, to make log files be more understandable?

This doesn't really answer your question, but it does circumvent your problem. bool(None) == False, but also bool('') == False. So in reality you just have to check if result: #pass, as '' and None both evaluate False. I'll try and find some reasoning for the choice of str(None) == 'None', but I don't think it should impact your code if you're just checking if a proper string was returned. — pswaminathan
– pswaminathan, Commented Jul 17, 2013 at 4:31
It's only extra work because the conditional you defined is awkward: "only deal with result if it's not None or its string representation is not empty". Use a more conventional way of checking (like suggested by @pswaminathan) and the problem goes away. — vanza
– vanza, Commented Jul 17, 2013 at 4:33
If foo returns None in case of failure, you need to check for None. In your example, anything whose string representation is empty is apparently handled the same was as None, but there's no particular reason why that should be the default behavior. The question is, why are you using "length of str(x)" as a diagnostic for anything? — BrenBarn
– BrenBarn, Commented Jul 17, 2013 at 4:34
Actually I think this is a very good question. It's easy to see why repr(None) should be 'None', but why shouldn't str(None) be ''? — Mark Ransom
– Mark Ransom, Commented Jul 17, 2013 at 4:37
@BobL: For the record, Python is not unique in this regard. For example, in Java, converting a null to a string results in the literal string "null". Reference: JLS 5.1.11. — Daniel Pryden
– Daniel Pryden, Commented Jul 17, 2013 at 4:39

Community · Accepted Answer · 2017-05-23 12:01:31Z

15

Checking if a string has characters in it by checking len(str(result)) is definitely not pythonic (see http://www.python.org/dev/peps/pep-0008/).

result = foo() # foo will return None if failure 
if result:
    # deal with result.
    pass

None and '' coerce to the boolean False.

If you are really asking why str(None) does return 'None', then I believe it is because it is necessary for three-valued logic. True, False and None can be used together to determine if a logical expression is True, False or cannot be decided. The identity function is the easiest for representation.

True  -> 'True'
False -> 'False'
None  -> 'None'

The following would be really weird if str(None) was '':

>>> or_statement = lambda a, b: "%s or %s = %s" % (a, b, a or b)
>>> or_statement(True, False)
'True or False = True'
>>> or_statement(True, None)
'True or None = True'
>>> or_statement(None, None)
'None or None = None'

Now, if you really want for an authoritative answer, ask Guido.

If you really want to have str(None) give you '' please read this other question: Python: most idiomatic way to convert None to empty string?

edited May 23, 2017 at 12:01

CommunityBot

11 silver badge

answered Jul 17, 2013 at 4:35

dnozay

24.4k6 gold badges85 silver badges105 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

GrantJ Over a year ago

If you need a complete implementation of three-valued logic that uses True, False and None then check out the Python Tribool module

Metaxis Over a year ago

Expanding on @GrantJ 's statement, or_statement() as written is not a good tri-state example because it has a different result depending on argument order: or_statement(None, False) should be undecidable, ie None but returns False because or is a simple shortcut operator that falls through after the boolean coercion of None to False.

BrenBarn · Accepted Answer · 2013-07-17 04:40:16Z

4

Basically, because an empty string isn't a representation of None. None is a special value that is distinct from an empty string or anything else. As described in the docs, str is supposed to

Return a string containing a nicely printable representation of an object.

Basically, str is supposed to return something printable and human-readable. An empty string would not be a readable representation of None.

answered Jul 17, 2013 at 4:40

BrenBarn

253k39 gold badges421 silver badges392 bronze badges

3 Comments

Mark Ransom Over a year ago

By that logic, str(None) and str('None') shouldn't be equal either.

DanielB Over a year ago

@Mark How does that suggest that str should have anything to do with equality? It's just a value that you can print. Use repr if you need to distinguish the type/value as a string.

Mark Ransom Over a year ago

@DanielB, I already expressed that sentiment in a comment on the question itself.

Collectives™ on Stack Overflow

Why python designed as str(None) return 'None' instead of an empty string?

2 Answers 2

2 Comments

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related