returning True, False or None

[Steven Bethard]

I have lists containing values that are all either True, False or None,
e.g.:

[True, None, None, False]
[None, False, False, None ]
[False, True, True, True ]
etc.

For a given list:
* If all values are None, the function should return None.
* If at least one value is True, the function should return True.
* Otherwise, the function should return False.

Right now, my code looks like:

if True in lst:
return True
elif False in lst:
return False
else:
return None

This has a light code smell for me though -- can anyone see a simpler
way of writing this?

STeVe

[Alex Martelli]

Steven Bethard <st************ @gmail.com> wrote:

I have lists containing values that are all either True, False or None,
e.g.:

[True, None, None, False]
[None, False, False, None ]
[False, True, True, True ]
etc.

For a given list:
* If all values are None, the function should return None.
* If at least one value is True, the function should return True.
* Otherwise, the function should return False.

Right now, my code looks like:

if True in lst:
return True
elif False in lst:
return False
else:
return None

This has a light code smell for me though -- can anyone see a simpler
way of writing this?

What about...:

for val in lst:
if val is not None:
return val
return None

or the somewhat fancy/clever:

for val in (x for x in lst if x is not None):
return val
return None
Alex

[Raymond Hettinger]

"Steven Bethard"

For a given list:
* If all values are None, the function should return None.
* If at least one value is True, the function should return True.
* Otherwise, the function should return False. . . . Right now, my code looks like:

if True in lst:
return True
elif False in lst:
return False
else:
return None

This has a light code smell for me though -- can anyone see a simpler
way of writing this?

return max(lst)
Raymond Hettinger

[Jeremy Bowers]

On Fri, 04 Feb 2005 10:48:44 -0700, Steven Bethard wrote:

I have lists containing values that are all either True, False or None,
e.g.:

[True, None, None, False]
[None, False, False, None ]
[False, True, True, True ]
etc.

For a given list:
* If all values are None, the function should return None.
* If at least one value is True, the function should return True.
* Otherwise, the function should return False.

Right now, my code looks like:

if True in lst:
return True
elif False in lst:
return False
else:
return None

Yes, I see the smell, you are searching the list multiple times. You
could bail out when you can:

seenFalse = False
for item in list:
if item: return True
if item is False: seenFalse = True
if seenFalse:
return False
return None

But I'd submit that if four item lists are your common case, that your
original code is significantly easier to understand what it is doing. This
can be alleviated with an appropriate comment on the chunk of code I gave
you, though.

[Steve Holden]

Steven Bethard wrote:

I have lists containing values that are all either True, False or None,
e.g.:

[True, None, None, False]
[None, False, False, None ]
[False, True, True, True ]
etc.

For a given list:
* If all values are None, the function should return None.
* If at least one value is True, the function should return True.
* Otherwise, the function should return False.

Right now, my code looks like:

if True in lst:
return True
elif False in lst:
return False
else:
return None

This has a light code smell for me though -- can anyone see a simpler
way of writing this?

STeVe

That code looks like a pretty solid implementation of the spec to me.
There isn't a strict need for the last else, of course, which may be the
smell you detect.

If you wanted to get clever you could write something like

for i in True, False:
if i in lst:
return i
return False

but frankly I think that's more obscure, and saves you pretty much nothing.

regards
Steve
--
Meet the Python developers and your c.l.py favorites March 23-25
Come to PyCon DC 2005 http://www.pycon.org/
Steve Holden http://www.holdenweb.com/

[Michael Spencer]

Steven Bethard wrote:

I have lists containing values that are all either True, False or None,
e.g.:

[True, None, None, False]
[None, False, False, None ]
[False, True, True, True ]
etc.

For a given list:
* If all values are None, the function should return None.
* If at least one value is True, the function should return True.
* Otherwise, the function should return False.

Right now, my code looks like:

if True in lst:
return True
elif False in lst:
return False
else:
return None

This has a light code smell for me though -- can anyone see a simpler
way of writing this?

STeVe

max(lst) ;-)

Michael

[Steven Bethard]

Raymond Hettinger wrote:

"Steven Bethard"

For a given list:
* If all values are None, the function should return None.
* If at least one value is True, the function should return True.
* Otherwise, the function should return False.

. . .

Right now, my code looks like:

if True in lst:
return True
elif False in lst:
return False
else:
return None

This has a light code smell for me though -- can anyone see a simpler
way of writing this?

return max(lst)

Very clever! Thanks!

Steve

[Mick Krippendorf]

Steven Bethard wrote:

I have lists containing values that are all either True, False or
None, e.g.:

[True, None, None, False]
[None, False, False, None ]
[False, True, True, True ]
etc.

For a given list:
* If all values are None, the function should return None.
* If at least one value is True, the function should return True.
* Otherwise, the function should return False.

Try:

max(lst)

Mick.

[Brian van den Broek]

Alex Martelli said unto the world upon 2005-02-04 13:02:

Steven Bethard <st************ @gmail.com> wrote:

I have lists containing values that are all either True, False or None,
e.g.:

[True, None, None, False]
[None, False, False, None ]
[False, True, True, True ]
etc.

For a given list:
* If all values are None, the function should return None.
* If at least one value is True, the function should return True.
* Otherwise, the function should return False.

Right now, my code looks like:
<SNIP OP's code>

This has a light code smell for me though -- can anyone see a simpler
way of writing this?

What about...:

for val in lst:
if val is not None:
return val
return None

or the somewhat fancy/clever:

for val in (x for x in lst if x is not None):
return val
return None
Alex

These don't do what the OP desired.

..>>> test_case = [False, True, True, True ]
..>>> def alexs_funct(lst ):
.. for val in lst:
.. if val is not None:
.. return val
.. return None

alexs_funct(tes t_case)

False

But, by the 'spec', it ought return True.

Best,

Brian vdB
A mere newbie, quite pleased with himself for finding a problem with
'bot code -- next scheduled to occur mid 2011 :-)

[Fredrik Lundh]

Steven Bethard wrote:

return max(lst)

Very clever! Thanks!

too clever. boolean > None isn't guaranteed by the language specification:

http://docs.python.org/ref/comparisons.html

"... objects of different types always compare unequal, and are ordered consistently
but arbitrarily. /.../ In the future, the comparison rules for objects of different types are
likely to change. ..."

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