python dictionary assignment using map and lambda

Question

I was just curious to know if the following loop is doable with the map() rather than using for loop ? If yes please be kind enough to show how ? or what's the most efficient way to do the following ?

f = open('sample_list.txt').read().split('\n')

val =   lambda x: re.match('[a-zA-z0-9]+',x).group() if x else None

for line in f:
    if line.find('XYZ') == -1:
        dict[val(line)]=0
    else:
        dict[val(line)]=1

This program reads a file formatted as :

ABCD XYZ 
DEFG ABC

then creates a dicionary with first word as KEY and if the second value is XYZ then it will assign a value of 1 else 0.

i.e dict will be :

{'ABCD': 1, 'DEFG': 0}

UPDATE :

I timed the approaches suggested by @shx2 and @arekolek dictionary comprehension is faster, and @arekolek's approach is way faster than anything as it doesn't use the val() lambda function i declared.

the code :

def a():
    return {
    val(line) : 0 if line.find('XYZ') == -1 else 1
    for line in f
    }

def b():
    return dict(map(lambda x: (x[0], int(x[1] == 'XYZ')), map(str.split, f)))

def c():
    return {k: int(v == 'XYZ') for k, v in map(str.split, f)}

print 'a='+timeit.timeit(a)
print 'b='+timeit.timeit(b)
print 'c='+timeit.timeit(c)

result :

a = 13.7737597283
b = 6.82668938135
c = 3.98859187269

Thanks both for showing this :)

jonrsharpe · Accepted Answer · 2016-06-27 10:22:55Z

5

Better to use dict comprehension than map.

Can be done like this:

my_dict = {
    val(line) : 0 if line.find('XYZ') == -1 else 1
    for line in f
}

Some notes and improvements:

dict is not a good variable name in python
Alternatives to the complicated expression 0 if line.find('XYZ') == -1 else 1:
- int(line.find('XYZ') != -1) (converting bool to int)
- if you can leave the values as bools: line.find('XYZ') != -1
- 'XYZ' in line (credit: @JonClements)

edited Jun 27, 2016 at 10:22

jonrsharpe

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answered Jun 27, 2016 at 10:09

shx2

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2 Comments

Jon Clements Over a year ago

Why use str.find at all here - if not 'XYZ' in line is more pythonic if you only want to check containment and aren't going to use the index...

CYAN CEVI Over a year ago

Thanks for your inputs, this is great.

arekolek · Accepted Answer · 2016-06-27 10:23:40Z

1

With a lambda:

dict(map(lambda x: (x[0], int(x[1] == 'XYZ')), map(str.split, f)))

Or:

dict(map(lambda k, v: (k, int(v == 'XYZ')), *map(str.split, f)))

Dictionary comprehension:

{k: int(v == 'XYZ') for k, v in map(str.split, f)}

edited Jun 27, 2016 at 10:23

answered Jun 27, 2016 at 10:18

arekolek

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