Subtract values from list of dictionaries from another list of dictionaries

Question

I have two lists of dictionaries.

foo = [{'Tom': 8.2}, {'Bob': 16.7}, {'Mike': 11.6}]
bar = [{'Tom': 4.2}, {'Bob': 6.7}, {'Mike': 10.2}]

The subtraction of a and b should be updated in foo:

foo = [{'Tom': 4.0}, {'Bob': 10.0}, {'Mike': 1.4}]

Now I tried this with two loops and the zip -function:

def sub(a,b):       
    for mydict,mydictcorr in zip(a,b):
        {k:[x-y for x, y in mydict[k], mydictcorr[k]] for k in mydict}
    return mydict
print sub(foo,bar)

I get a TypeError: 'float' object is not iterable . Where's my mistake?

Answer 1

You were very close. The issue was the list comprehension you had in your dictionary comprehension. mydict[k], mydictcorr[k] were both returning floats, but you were trying to iterate over them [xy for x, y in mydict[k], mydictcorr[k]] .

This will work for you:

def sub(base, subtract):
    corrected = []
    for base_dict, sub_dict in zip(base, subtract):
        corrected.append({k: v - sub_dict.get(k, 0) for k, v in base_dict.items()})
    return corrected

Or as a much less readable one-liner (because I wanted to see if I could):

def sub(base, subtract):
    return [{k: v - sub_dict.get(k, 0) for k, v in base_dict.items()} for base_dict, sub_dict in zip(base, subtract)]

Having said that, you may still see some weird results when you subtract floats. Eg, {'Tom': 3.999999999999999} . You may want to wrap v - sub_dict.get(k, 0) in a call to round.

Answer 2

You can build dictionaries from foo and bar and use list comprehension:

a = dict([next(x.iteritems()) for x in foo])    
b = dict([next(x.iteritems()) for x in bar])

sub_dict = dict([(key, round(a[key] - b.get(key, 0), 1)) for key in a])

Output:

{'Bob': 10.0, 'Mike': 1.4, 'Tom': 4.0}

Answer 3

If you want the dicts in foo updated you can do it directly as lists and dicts are both immutable:

def sub(a, b):
    for d1, d2 in zip(a, b):
        for k, v in d2.items():
            d1[k] -= v

Any changes will be reflected in your list which just holds references to your dicst:

In [2]: foo
Out[2]: [{'Tom': 8.2}, {'Bob': 16.7}, {'Mike': 11.6}]

In [3]: sub(foo, bar)

In [4]: foo
Out[4]: [{'Tom': 3.999999999999999}, {'Bob': 10.0}, {'Mike': 1.4000000000000004}]

As you can see you also need to be aware of floating-point-arithmetic-issues-and-limitations