How to test whether two lists are equal in python

Question

The following script is returning false, when I think that it should be returning true. Any idea what is going on here? Thanks so much, guys!

test=['Pop']
test1='Pop'

if (test==('POP' or 'Pop' or 'pop' or ['POP'] or ['Pop'] or ['pop'])):    
    print "yes"
else:
    print "no"

Currently, the output is, 'no'.

You aren't comparing two lists, you are comparing a list to a string. — Levon
– Levon, Commented Jun 27, 2012 at 17:32
if test == 'POP' or '...' or '...' isn't doing what you think it is: possibly have a look in the fine documentation how boolean logic works — Jon Clements
– Jon Clements, Commented Jun 27, 2012 at 17:34
To ignore case of a string, you can convert both operands to lowercase. 'POP'.lower() == 'poP'.lower() — Niklas R
– Niklas R, Commented Jun 27, 2012 at 17:35
Then I strongly suggest reading the Python tutorial - if you're struggling and don't understand what everyone's said - you're going to run into serious problems later on — Jon Clements
– Jon Clements, Commented Jun 27, 2012 at 17:46

Winston Ewert · Accepted Answer · 2012-06-27 17:46:37Z

7

You aren't understanding how python processes the statement. Python isn't natural language.

if (test==(['Pop'] or 'Pop')):

Because the or is inside the parens, it processes it first. So it looks at

['Pop'] or 'Pop'

Since ['Pop'] is considered True, python reduces the whole statement to:

if (test==['Pop']):

At this point, it tests whether test is equal to ['Pop']

What you are actually wanting to do is:

(test == ['Pop']) or (test == 'Pop')

This is completely different than

 test == (['Pop'] or 'Pop')

answered Jun 27, 2012 at 17:46

Winston Ewert

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4 Comments

Jon Clements Over a year ago

Or more traditionally written as test in (['Pop'], 'Pop')

Winston Ewert Over a year ago

@JonClements, I don't know about traditionally, but yes that's better.

Atticus29 Over a year ago

Thank you. This was exactly my confusion. Thank you for being able to identify this despite the shifting code above.

Amr Over a year ago

@Atticus29: also note that unless you're using your own class which has it's own comparison methods, equality tests would evaluate to False between objects of different types no matter what their contents are.

Simeon Visser · Accepted Answer · 2012-06-27 17:33:54Z

2

If you write it like this, the if statement will be true when you use test or test1:

test=['Pop']
test1='Pop'
if (test in ('POP', 'Pop', 'pop', ['POP'], ['Pop'], ['pop'])):
    print "yes"
else:
    print "no"

You're basically creating a big tuple with all the possibilities: three strings and three lists. If your variable is present in there, the if statement is true.

answered Jun 27, 2012 at 17:33

Simeon Visser

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5 Comments

JAB Over a year ago

I believe if any(test == i for i in ('POP', 'Pop', 'pop', ['POP'], ['Pop'], ['pop'])): would also work.

Winston Ewert Over a year ago

@JAB, yes, but why would ever want to do it that way?

notbad.jpeg Over a year ago

This doesn't answer the question.

Winston Ewert Over a year ago

this is the right way of doing it, but it would better to explain what's wrong with the OP's approach.

JAB Over a year ago

@WinstonEwert So you can utilize a generator expression just for the heck of it. As an aside, Simeon didn't need to use parentheses with the if statement in C/Java style; if test in ('POP', 'Pop', 'pop', ['POP'], ['Pop'], ['pop']): has the same result.

jfs · Accepted Answer · 2012-06-27 17:59:15Z

1

test == (a or b) is different from test == a or test == b.

(a or b) returns a iff bool(a) is True and b otherwise. Therefore test == ('POP' or whatever) is equivalent to test == 'POP' because bool('POP') is True as for any other non-empty string in Python.

To test multiple values you could use: value in [a, b].

Note: ['a'] != 'a' – the later is a string while the former is a list that contains a string.

In your case you could test[0].lower() == 'pop'.

answered Jun 27, 2012 at 17:59

jfs

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Comments

underbar · Accepted Answer · 2012-06-27 17:44:10Z

0

test=['Pop']
test1='Pop'
if test1 == 'POP' or test1 == 'Pop' or test1 == 'pop' or test1 == ['POP'] or test1 == ['Pop'] or test1 == ['pop']:
    print "yes"

else:
    print "no"

Each one is a separate test.

edited Jun 27, 2012 at 17:44

answered Jun 27, 2012 at 17:36

underbar

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3 Comments

notbad.jpeg Over a year ago

This is the closest answer to your question. The other ones are saying something completely different.

Jon Clements Over a year ago

This always results in if test1 == 'POP'

Winston Ewert Over a year ago

This is just completely wrong. It will always say "yes" regardless of what's in test1

Stuart Axon · Accepted Answer · 2012-11-16 10:01:54Z

0

it looks like the user is just trying to do a case insensitive comparison:

if test1.lower() == 'pop':
    print 'yes'
else:
    print 'no'

answered Nov 16, 2012 at 10:01

Stuart Axon

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