Howcome a string type variable is considered as boolean in Python?

Question

In the code below:

def modify_note(self):
    id = input("Enter a note id: ")
    memo = input("Enter a memo: ")
    tags = input("Enter tags: ")
    if memo:
       self.notebook.modify_memo(id, memo)
    if tags:
       self.notebook.modify_tags(id, tags)

memo and tags are string type variables. How can you write them after if, does python regard them as boolean here?

Is this Python 3.x? input() has a different meaning in 2.x and 3.x. — Sven Marnach
– Sven Marnach, Commented May 13, 2011 at 13:51
Just a note: unless you're using python 3, careful with input()! It's not the same as raw_input, as it evaluates whatever input you type in, equivalent to eval(raw_input())! — bluepnume
– bluepnume, Commented May 13, 2011 at 13:53
yes it is python 3.x. what happened to raw_input in python 3? — alwbtc
– alwbtc, Commented May 13, 2011 at 14:08
"what happened to raw_input in python 3?" That's an unrelated question. First, search stack overflow. Then read the Python 3 notes in Python's web site. It's no secret. Please read all available information. — S.Lott
– S.Lott, Commented May 13, 2011 at 14:27

Sven Marnach · Accepted Answer · 2011-05-13 13:51:04Z

8

Every object in Python has a truth value. Strings are True if they are non-empty.

answered May 13, 2011 at 13:51

Sven Marnach

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Matt Ball · Accepted Answer · 2011-05-13 13:51:22Z

6

The if memo and if tags statements are checking the truthiness of the memo and tags variables.

Any object can be tested for truth value, for use in an if or while condition or as operand of the Boolean operations below. The following values are considered false:

None

False

zero of any numeric type, for example, 0, 0L, 0.0, 0j.

any empty sequence, for example, '', (), [].

any empty mapping, for example, {}.

instances of user-defined classes, if the class defines a __nonzero__() or __len__() method, when that method returns the integer zero or bool value False.

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

answered May 13, 2011 at 13:51

Matt Ball

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2 Comments

alwbtc Over a year ago

but in that code there is no way entering an empty string for tags and memo variables. which means they will always be true. right?

user395760 Over a year ago

@alwbtc: If the user hits enter right away, you get an empty string, as the newline isn't included in the return value.

planetjones · Accepted Answer · 2011-05-13 13:51:57Z

0

It all depends on Python's version of truthiness

answered May 13, 2011 at 13:51

planetjones

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