Python: Identifying string list with length of 1 vs. string

Question

In my code I have the following:

if all(requiredField in submittedFields for requiredField in requiredFields):
    # all required fields in submittedFields exist
else:
    # error handling

The goal is to check if a list of strings in requiredFields are all present in submittedFields

This works fine when requiredFields is a list of strings with length > 1. But, when you have something like

requiredFields = ('single element')

Then the for loop iterates over each character instead of the string itself.

So my question is, is there a more pythonic means of handling this other than

try: 
    requiredFields.sort()
    # requiredFields is a list of strings
except AttributeError:
    # requiredFields is a string list whose length == 1

Raymond Hettinger · Accepted Answer · 2011-11-23 19:55:56Z

6

Using python sets would be much more efficient:

submitted_fields = set(['length', 'width', 'color', 'single element'])
required_fields = set(['width', 'length'])
if submitted_fields >= required_fields:
    # all required fields in submittedFields exist
else:
    # error handling

Several optimizations make this fast:

The hash table implementation of sets ensures a high likelihood of a match before doing a character by character equality test.
If two strings are identical (the same object in memory), the identity check will bypass a character-by-character equality check.

Note. It looks like your original problem was with tuple notation. Steven Rumbalski addressed that very well. Of course, if you use sets, this becomes a non-issue.

Good luck with your field validation :-)

edited Nov 23, 2011 at 19:55

answered Nov 23, 2011 at 19:50

Raymond Hettinger

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5 Comments

Spencer Rathbun Over a year ago

-1 care to provide an example? I know it's faster, but you haven't actually answered his question, or provided an alternative.

user297250 Over a year ago

Yes, it will be orders of magnitudes faster.

Shawn Chin Over a year ago

@SpencerRathbun While not immediately obvious, it does answer the question. Specifically the "is there a more pythonic means of handling this" bit. The question is what one would call an XY problem

Spencer Rathbun Over a year ago

@ShawnChin True, but I could just as easily say that "a more efficient solution would be to write a function in assembler. This is left as an exercise to the reader..." Which is why I gave him the -1. Since he added an example, I'm changing that to a +1.

Shawn Chin Over a year ago

@SpencerRathbun If you had posted that, then it WOULD warrant a -1. >;)

Community · Accepted Answer · 2017-05-23 12:03:37Z

5

A string enclosed in parenthesis is not a tuple -- it's a string. To make a one item tuple you need a trailing comma:

>>> ('single element') # this is not a tuple, it's a string
'single element'
>>> ('single element',) # note the trailing comma
('single element',)

For more info see the wiki on Tuple Syntax the Stack Overflow question Python tuple comma syntax rule.

edited May 23, 2017 at 12:03

CommunityBot

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answered Nov 23, 2011 at 19:49

Steven Rumbalski

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Comments

simon · Accepted Answer · 2011-11-23 19:51:17Z

0

first of all you should never have requiredFields = ('single element') -- if you want a tuple you should write requiredFields = ('single element',).

but assuming you don't have control over your input, the most pythonic way to test is:

if isinstance(requiredFields, basestring):
    # requiredFields is a string
else:
    # requiredFields is iterable

answered Nov 23, 2011 at 19:51

simon

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Comments

icirellik · Accepted Answer · 2011-11-23 19:56:41Z

0

Why don't you just do something like this:

required = ['name', 'password']
submitted = ['name', 'email']

for field in required:
    if field not in submitted:
        print field + ' is required'

answered Nov 23, 2011 at 19:56

icirellik

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Comments

agf · Accepted Answer · 2012-04-21 14:46:21Z

0

This happens because the definition

requiredFields = ('single element')

is actually equivalent to

requiredFields = 'single element'

To make a single element list, do

requiredFields = ['single element']

To make a single element tuple, do

requiredFields = ('single element',)

edited Apr 21, 2012 at 14:46

agf

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answered Nov 23, 2011 at 19:50

infrared

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1 Comment

agf Over a year ago

If you're going to put just code on a line, indent it four spaces or use the {} button rather than using backticks.

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