CODEWITHSUNDEEP
pythondictionarydictionary-comprehension
jasonC
jasonC

Reputation: 347

Difference between Python dictionary comprehension and loop

I am using Python 3.4, and I am testing out dictionary comprehension.

Let's say I have the following code:

listofdict = [{"id":1, "title": "asc", "section": "123"},{"id":2, "title": "ewr", "section": "456"}]
titles1 = []
titles2 = []
titles1.append({r["section"]: r["title"] for r in listofdict})
print("titles1 = " + str(titles1))

for r in listofdict:
  section = r["section"]
  title = r["title"]
  titles2.append({section: title})

print("titles2 = " + str(titles2))

I thought both methods should give me the same result, but I get the following instead:

titles1 = [{'456': 'ewr', '123': 'asc'}]
titles2 = [{'123': 'asc'}, {'456': 'ewr'}]

titles2 is what I actually want, but I want to use dictionary comprehension to do it.

What is the right way to write the dictionary comprehension?

Upvotes: 1

Views: 339

Answers (1)

Martijn Pieters
Martijn Pieters

Reputation: 1123400

You cannot use a dict comprehension for that, because a dict comprehension produces one dictionary with the keys and values taken from the loop(s).

You'd use a list comprehension instead:

[{r["section"]: r["title"]} for r in listofdict]

This produces the one dictionary each iteration, producing a new list:

>>> listofdict = [{"id":1, "title": "asc", "section": "123"},{"id":2, "title": "ewr", "section": "456"}]
>>> [{r["section"]: r["title"]} for r in listofdict]
[{'123': 'asc'}, {'456': 'ewr'}]

Upvotes: 8

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