What's the logical value of "string" in Python?

Question

I erroneously wrote this code in Python:

name = input("what is your name?")
if name == "Kamran" or "Samaneh":
    print("That is a nice name")
else:
    print("You have a boring name ;)")

It always prints out "That is a nice name" even when the input is neither "Kamran" nor "Samaneh".

Am I correct in saying that it considers "Samaneh" as a true? Why?

By the way, I already noticed my mistake. The correct form is:

if name == "Kamran" or name == "Samaneh":

Solution 1

Any non empty string in Python (and most other languages) is true as are all non-zero numbers and non-empty lists, dictionaries, sets and tuples.¹

A nicer way to do what you want is:

name = input("what is your name?")
if name in ("Kamran", "Samaneh"):
    print("That is a nice name")
else:
    print("You have a boring name ;)")

This creates a tuple containing the names that you want and performs a membership test.

^{1 As delnan points out in the comments, this applies to all well written collections. That is, if you implement a custom collection class, make sure that it is false when it's empty.}

Solution 2

Besides the empty string '', strings will all evaluate to True (see this page for a full list of values of all types that evaluate to False. This follows the logic of many other programming languages (except some which also evaluate strings like '0', 'false', etc. to False). The exact decision of what to do is somewhat arbitrary, but the choice made can be explained as allowing the cast to be used as a simple way to test for empty (default, or unpopulated) strings.

You can always force a cast of any type to bool using the bool() function.

>>> bool('')
False
>>> bool('non-empty string')
True
>>> bool('0')
True
>>> bool('False')
True
>>> bool('false')
True

Solution 3

http://docs.python.org/library/stdtypes.html#truth-value-testing

"....All other values are considered true — so objects of many types are always true."

Solution 4

In Python an empty string is considered False, True otherwise.

You could use the in operator:

if name in ("Kamran","Samaneh"):
    print("That is a nice name")
else:
    print("You have a boring name ;)")

View more solutions

Comments

[Admin]

Not only lists, dicts and tuples -- all empty collections (if well-written).

[aaronasterling]

@delnan, I was trying to think of a good way to say that so I stuck to the built ins and forgot sets.

[Kabie]

There is no raw_input in Python3 and if name == ("Kamran" or "Samaneh") is just wrong as if (name == "Kamran" or "Samaneh").

[Kamran Bigdely]

I tested name == ("Kamran" or "Samaneh"). When I enter "Samaneh" it gives me "You have a boring name ;)" which is not correct.

[eyquem]

I absolutely understand nothing why I posted such a false and stupid answer. Despite the fact I had studied this matter of chains considered as booleans in Python some time ago, and I thought I had well understood the thing, I completely screwed up.

[eyquem]

Usually, I always test a script before I post. Here, it seems I didn’t, since the expression name == ("Kamran" or "Samaneh") gives True only when name is ’Kamran’ and therefore can’t be used to detect when name has the value ’Samaneh“.

[eyquem]

The fact is that ("Kamran" or "Samaneh") is always “Kamran“ and that’s not particularly difficult to see. I hope Kamran edited his question only after I read it, because now it is well pointed out in his question that the correct form is if name == "Kamran" or name == "Samaneh": and if this would have been already in the question when I read it, it would be even more stupid for me not to have been alerted.

[eyquem]

I am not pleased to have received downvotes but they are justified. I don’t understand what I got up to with this problem, I think it was late and I was tired.

[eyquem]

PS I had problems to post this comment in only one chunk. I don’t know if it’s because of my out-to-date operating system that displays pages weirdly, or if the normal working of stackoverflow. OS or SO ? :)

[Daniel H]

Most objects, collection or otherwise, are truthy; only the ones wheich are explicitly define otherwise aren’t. If you just define a class with class C(): pass, instances will test as True.