Dictionary: replacing key characters with whitespace

Question

I have a dictionary that looks like this:

result =  {'Mars': 1358996659, 'Snickers': 26100257, 'Ben%and%Jerry's': 102656, 'Tootsie%roll': 3825986}

I want to remove the '%' and replace it with a whitespace instead.

I've tried iterating over the list like so:

for key in result:
if "%" in key:
    key.replace("%"," ")
    print(key)

But it didn't work. I've also experimented with other loops, but nothing seems to work for me.

I would like to end up with a dictionary that looks like this:

result =  {'Mars': 1358996659, 'Snickers': 26100257, 'Ben and Jerry's': 102656, 'Tootsie roll': 3825986}

You have several good answers. But for future reference, remember that Python strings are immutable, thus string methods like `.replace` return a new string since they cannot affect the original. Also, there's no need to do the `if "%" in key` since `.replace` will do that test internally, and more efficiently. — PM 2Ring, May 18 '18 at 11:15

Martijn Pieters · Accepted Answer · 2018-05-18T11:28:35.497

Use a dictionary comprehension to build a new dictionary, so you can replace the keys in one go:

result = {key.replace('%', ' '): value for key, value in result.items()}

The comprehension consists of:

an expression to generate a key, the part before the :
an expression to generate a value, the part after : but before the for
a for loop; you'll get as many key-value pairs as the for loop iterates (unless you add more for loops or if filters).

Your own solution didn't work, because str.replace() creates a new string object and returns that. The old string is never changed, because strings are immutable; they can't be altered in-place. If you wanted to do the work in a loop, you'd have to:

store the result of the str.replace() operation.
remove the old key (storing the value that it referenced temporarily)
add the new key to the dictionary, making it point to the value for the old key.

However, looping over a dictionary while mutating it (deleting and adding keys is mutating), is not safe and thus not supported. You'd have to loop over a copy of the keys; create one with list(result):

for key in list(result):
    if '%' in key:
        new_key = key.replace('%', ' ')
        value = result.pop(key)  # remove the old key, store the value
        result[new_key] = value

Re-generating the dictionary with a dictionary comprehension is just easier, at this point.

Note that in both approaches, you can end up with fewer keys if replacing '%' with ' ' leads to collisions! Which value remains after processing depends on which now-duplicate key was processed last.

score 0 · Answer 2 · answered May 18 '18 at 11:06

0

You can't change dictionary keys in place. You have to create a new dictionary, at least temporarily.
This: key.replace("%"," ") doesn't do what you think it does. It doesn't change key, it returns a new value.

I had to fix your data, like this:

>>> result =  {'Mars': 1358996659, 'Snickers': 26100257, "Ben%and%Jerry's": 102656, 'Tootsie%roll': 3825986}

Then:

>>> result2 = {}
>>> for key, value in result.items():
        result2[key.replace("%"," ")] = value
>>> result2
{'Tootsie roll': 3825986, "Ben and Jerry's": 102656, 'Snickers': 26100257, 'Mars': 1358996659}

answered May 18 '18 at 11:06

BoarGules

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You do not *have* to create a new dictionary. You can update the dictionary in-place, provided you create a copy of keys first. – Martijn Pieters May 18 '18 at 11:29
And isn't that at least a partial copy of the dictionary? And what about collisions? – BoarGules May 18 '18 at 11:30
It's a list with references to the keys, so *technically*, perhaps yes, but it is *not a new dictionary*. – Martijn Pieters May 18 '18 at 11:31
Collisions are not an issue. You get fewer keys in the end. – Martijn Pieters May 18 '18 at 11:31
Umm, see if you can explain that in 4 lines of code to a novice who doesn't know how `str.replace()` works. – BoarGules May 18 '18 at 11:32
Collisions are an issue. In the general case you might get keys that exchange values. – BoarGules May 18 '18 at 11:34
I'm not sure why that's a problem? My answer uses one line of code, plus further explanations on how their solution didn't work, with 5 lines that can be reduced to 4 if you inline the `new_key` value in to the assignment. – Martijn Pieters May 18 '18 at 11:34
Sure, collisions need to be *mentioned*, but they don't require a new dictionary to solve. – Martijn Pieters May 18 '18 at 11:35

score 0 · Answer 3 · answered May 18 '18 at 12:21

The code below will store the replaced key in a variable and you can add it to the dictionary, while removing the old key and value from the dictionary. The following code should give you the output that you are looking for:

result =  {'Mars': 1358996659, 'Snickers': 26100257, "Ben%and%Jerry's": 102656, 'Tootsie%roll': 3825986}

for key, value in result.items():
    if '%' in key:
       new_key = key.replace('%', ' ')
       del result[key]
       result[new_key] = value
print(result)

And here is your output:

{'Mars': 1358996659, 'Snickers': 26100257, "Ben and Jerry's": 102656, 'Tootsie roll': 3825986}

Dictionary: replacing key characters with whitespace

3 Answers3