Empty string is False right?

[AJ Ostergaard]

Hello,

First post so bear with me if I'm being a numpty ...

Is it me or is there something slightly counter intuitive and thus not
so pythonesque about this:
.... else: False
....
False
Regards,
AJ

 

[Ralf Schoenian]

Hello,

First post so bear with me if I'm being a numpty ...

Is it me or is there something slightly counter intuitive and thus not
so pythonesque about this:

.... else: False
....
False

Regards,
AJ

Hi,

yes, the following evaluates to False:
empty String: ''
empty list: []
empty tuple: ()
empty dict: {}
0, None
and False of course

Regards,
Ralf

 

[Ralf Schoenian]

Hello,

First post so bear with me if I'm being a numpty ...

Is it me or is there something slightly counter intuitive and thus not
so pythonesque about this:

.... else: False
....
False

Regards,
AJ

Hi,

yes, the following evaluates to False:
empty String: ''
empty list: []
empty tuple: ()
empty dict: {}
0, None
and False of course

Regards,
Ralf

 

[AJ Ostergaard]

Hi Ralf,

Thanks for that but why:
''

Surely that should be False?!?

Regards,
AJ

Hello,
First post so bear with me if I'm being a numpty ...
Is it me or is there something slightly counter intuitive and thus
not so pythonesque about this:
.... else: False
....
False
Regards,
AJ

Hi,

yes, the following evaluates to False:
empty String: ''
empty list: []
empty tuple: ()
empty dict: {}
0, None
and False of course

Regards,
Ralf

 

[Gabriel Genellina]

Hi Ralf,

Thanks for that but why:

''

Surely that should be False?!?

Python does "short-circuit evaluation" [1]
"and" and "or" return one of its operands as soon as the outcome is
determined, not just True or False.
'' is a false value, as false as False itself
After seeing that, there is no point in evaluating the second operand
(True) because the final result cannot be true; so Python just returns the
first operand.

[1] http://en.wikipedia.org/wiki/Short_circuit_evaluation

 

[Bernd Nawothnig]

Thanks for that but why:

Surely that should be False?!?

It is:

#v+False
#v-



Bernd

 

[Vlastimil Brom]

Hi Ralf,

Thanks for that but why:

''

Surely that should be False?!?

Regards,
AJ

see the docs:
http://docs.python.org/reference/expressions.html#boolean-operations

"The expression x and y first evaluates x; if x is false, its value is
returned; otherwise, y is evaluated and the resulting value is
returned."

hth,
vbr

 

[John Machin]

yes, the following evaluates to False:

A much better way of describing the effect would be to say that the
following are treated as false (no capital letter!) in a conditional
context.

empty String: ''
empty list: []
empty tuple: ()
empty dict: {}

*any* empty container ...

0, None
and False of course

and objects which adhere to the protocol

 

[AJ Ostergaard]

I'm not suggesting it's not operating as advertised - I'm suggesting
the 'advertising' is slightly sguiffy if you catch my drift. I guess
it's just me that finds it slightly counter intuitive. Surely
intuitively the expression is "and" and therefore should always return
a boolean?

I'll shut up now.

AJ

 

[John Machin]

Hi Ralf,

Thanks for that but why:

 >>> '' and True
''

Surely that should be False?!?

No, deliberately not. Read this for Python 3.0
http://docs.python.org/3.0/reference/expressions.html#boolean-operations
and/or this for Python 2.X
http://docs.python.org/reference/expressions.html#boolean-operations
[essentially the only difference in 3.0 is a change to the protocol
that allows a non-builtin object to say whether it is true or false]

 

[D'Arcy J.M. Cain]

''

Surely that should be False?!?

Why? The first value evaluates to False in a boolean context and
thus is returned in the above statement due to short circuit
evaluation but is not itself False. You wouldn't expect the following
statement to be True.
False

 

[Steve Holden]

I'm not suggesting it's not operating as advertised - I'm suggesting the
'advertising' is slightly sguiffy if you catch my drift. I guess it's
just me that finds it slightly counter intuitive. Surely intuitively the
expression is "and" and therefore should always return a boolean?

I'll shut up now.

You might think so, and it wouldn't be an entirely unreasonable thought,
but in practice it makes a lot of sense to retain the original value
where possible.

The fact is that any left-hand operand that evaluates to false in a
Boolean context can be used as it stands rather than being converted to
Boolean first. So the conversion is essentially useless processing.

regards
Steve

 

[Diez B. Roggisch]

Hi Ralf,

Thanks for that but why:

''

Surely that should be False?!?

No. Please read the section in the language reference about the and/or
operators.

"and" will return the first false value, or the right side. Thus

'' and True -> ''

True and '' -> ''

'a' and True -> True

True and 'a' -> 'a'

"or" does the same, obviously with the or-semantics:


'' or False -> False
False or '' -> ''
'' or True -> True
True or '' -> True
'a' or False -> 'a'
False or 'a' -> 'a'

Diez