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":

Asked By: Kamran Bigdely

||

Source

Answer 1

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

“….All other values are considered true — so objects of many types are always true.”

Answered By: Uku Loskit

Answer 2

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.}

Answered By: aaronasterling

Answer 3

A non-empty string is True, yes. An empty one is False. This is super-handy.

Answered By: Lennart Regebro

Answer 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 ;)")

Answered By: systempuntoout

Answer 5

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

Answered By: moinudin

Answer 6

Evidently, it must be:

name = raw_input("what is your name?")

not

name = input("what is your name?")

.

What you wrote is like:

if (name == "Kamran" or "Samaneh")

Another good expression is:

if name == ("Kamran" or "Samaneh")

But I also prefer if name in (“Kamran” or “Samaneh”) as already shown

.

By the way, it can be written:

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

Answered By: eyquem

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

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