python basic syntax boolean compare

Question

i have a question about the following code

 smaller={}
 for( dest in a[neigbour].keys())

    if(dest in smaller.keys() == False):
        print 'false'
    }

I can't make this code print false.. am I doing something wrong? I wonder if I am doing the correct thing to check the statement dest in smaller.keys() == False

You should never compare with False or True in conditional statements. — Pavel Paulau
– Pavel Paulau, Commented Oct 21, 2012 at 23:44
If you're testing membership in a dictionary, you don't need x in d.keys(), you can simply use x in d. Similarly, for x in d: loops over the keys. — DSM
– DSM, Commented Oct 21, 2012 at 23:48

mathematical.coffee · Accepted Answer · 2012-10-21 23:45:44Z

4

The oppisite of dest in smaller.keys() is dest not in smaller.keys(). No need to compare to False or True:

if (dest not in smaller.keys()):

Documentation for in and not in: http://docs.python.org/library/stdtypes.html#sequence-types-str-unicode-list-tuple-bytearray-buffer-xrange

answered Oct 21, 2012 at 23:45

mathematical.coffee

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Comments

Yuushi · Accepted Answer · 2012-10-21 23:46:15Z

4

Your Python syntax is quite confused. For one, you need a : after your for statement, and it's generally not idiomatic to use braces around a for loop in Python. Also, instead of comparing to False with ==, generally we use not:

smaller = {}
for dest in a[neighbour].keys():
    if dest not in smaller.keys():
        print('false')

answered Oct 21, 2012 at 23:46

Yuushi

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Comments

Jon Clements · Accepted Answer · 2012-10-22 09:20:37Z

1

As well as the other answers you've been given, the code could be written as:

for key in a[neighbour].viewkeys() - smaller.viewkeys():
    print key, 'not found'

Which takes advantage of the set like behaviour of .viewkeys to easily create a set of all keys in a[neighbour] not in b, then loops over that.

answered Oct 22, 2012 at 9:20

Jon Clements

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