How to create a dictionary from a list with a list of keys only and key-value pairs (Python)?

Question

This is an extension of this question: How to split a string within a list to create key-value pairs in Python

The difference from the above question is the items in my list are not all key-value pairs; some items need to be assigned a value.

I have a list:

list = ['abc=ddd', 'ef', 'ghj', 'jkl=yui', 'rty']

I would like to create a dictionary:

dict = { 'abc':'ddd', 'ef':1, 'ghj':1, 'jkl':'yui', 'rty':1 }

I was thinking something along the lines of:

a = {}
for item in list:
   if '=' in item: 
     d = item.split('=')
     a.append(d) #I don't I can do this.
   else:
     a[item] = 1 #feel like I'm missing something here.

you need some parser logic, which detects list elements containing '=' and splits those into key and value. — nucleon, Jun 08 '16 at 14:25

score 13 · Accepted Answer · answered Jun 08 '16 at 14:31

13

For each split "pair", you can append [1] and extract the first 2 elements. This way, 1 will be used when there isn't a value:

print dict((s.split('=')+[1])[:2] for s in l)

answered Jun 08 '16 at 14:31

Fabricator

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This is a stunner. Brilliant! – Moses Koledoye Jun 08 '16 at 14:41
thanks @fabricator! Could explain why it doesn't work if you don't specifically extract the first two elements `[:2]`? Want to make sure I understand everything. – Amelia N Chu Jun 08 '16 at 14:45
1

@AmeliaNChu, just because `dict` method requires it. from the [documentation](https://docs.python.org/2/library/stdtypes.html#dict): `Otherwise, the positional argument must be an iterable object. Each item in the iterable must itself be an iterable with exactly two objects` – Fabricator Jun 08 '16 at 14:49
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Note, will fail silently if there are ever two or more "=" in s. (It'll truncate at the second "="). – nigel222 Jun 08 '16 at 15:41

score 3 · Answer 2 · edited May 23 '17 at 10:33

3

I would be using something similar to the post you linked.

d = dict(s.split('=') for s in a)

If you combine what you can learn from this post -- that is to use lists to create dictionaries -- and from using if/else in Python's list comprehension, you can come up with something like this:

d = dict(s.split("=", maxsplit=1) if "=" in s else [s, 1] for s in l)

What this does is add 1 to the end of the split list if there is no equal sign in it.

edited May 23 '17 at 10:33

Community

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answered Jun 08 '16 at 14:37

Moon Cheesez

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`[s]+[1]` can be shorten into `[s, 1]` – Hai Vu Jun 08 '16 at 14:38
This is the real stunner in my books - one line *and* completely readable. – nigel222 Jun 08 '16 at 15:25
Note, will crash (raise ValueError) if there are ever two or more "=" in `s`. Which may well be the best thing to do. – nigel222 Jun 08 '16 at 15:39
1

If you want to crash-proof it use `s.split("=", maxsplit=1)`. I had to look that up. But as soon as I thought of the need, I suspected that the split method must have provided for it, and it had. Which is why I love Python. – nigel222 Jun 08 '16 at 15:56

saq7 · Answer 3 · 2016-06-08T14:45:23.537

1

input_list = ['abc=ddd', 'ef', 'ghj', 'jkl=yui', 'rty']
output_dict = {}

for item in input_list:
    item_split = item.split('=')
    key = item_split[0]
    value = item_split[1] if len(item_split)>1 else 1
    output_dict[key] = value

a bit more concisely

for item in input_list:
    i_s = item.split('=')
    output_dict[i_s[0]] = i_s[1] if len(i_s)>1 else 1

This has the advantage that it doesn't append an extra element to each list created by splitting the elements of the input_list. Though, list comprehensions can be faster than a for loop

edited Jun 08 '16 at 14:45

answered Jun 08 '16 at 14:24

saq7

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It can be done much more easily, but I would wait for the code tried and errors faced first before I post it. – Moon Cheesez Jun 08 '16 at 14:27

score 1 · Answer 4 · answered Jun 08 '16 at 14:37

Here are the step-by-step approach.

In [50]: mylist = ['abc=ddd', 'ef', 'ghj', 'jkl=yui', 'rty']

In [51]: [element.split('=') for element in mylist]
Out[51]: [['abc', 'ddd'], ['ef'], ['ghj'], ['jkl', 'yui'], ['rty']]

In [52]: [element.split('=') + [1] for element in mylist]
Out[52]: [['abc', 'ddd', 1], ['ef', 1], ['ghj', 1], ['jkl', 'yui', 1], ['rty', 1]]

In [53]: [(element.split('=') + [1])[:2] for element in mylist]
Out[53]: [['abc', 'ddd'], ['ef', 1], ['ghj', 1], ['jkl', 'yui'], ['rty', 1]]

In [54]: dict((element.split('=') + [1])[:2] for element in mylist)
Out[54]: {'abc': 'ddd', 'ef': 1, 'ghj': 1, 'jkl': 'yui', 'rty': 1}

In order to convert your list in line 50 to a dictionary, you will need to convert it to the list in line 53.
Line 51. The first step is to split each element in the list by the equal sign. Each element now is transformed into a list of 1- or 2 elements. Notice that some element like 'ef' which does not have equal sign, we will have to fix that
Line 52. Next, we append 1 to each sub list. That should take care of the sublists with 1 element, but making some sublist 3 element long
Line 53: We normalize all sublist into 2-element ones by taking just the first two element and discard the third one if applicable. This list now is in the correct format to convert into a dictionary
Line 54. The last step is to take this list and convert it into a dictionary. Since the dict class can take a generator expression, we can safely remove the square brackets.

With that, here is the snippet:

mylist = ['abc=ddd', 'ef', 'ghj', 'jkl=yui', 'rty']
mydict = dict((element.split('=') + [1])[:2] for element in mylist)

How to create a dictionary from a list with a list of keys only and key-value pairs (Python)?

4 Answers4

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