Should I write dict or {} in Python when constructing a dictionary with string keys?

Question

This is just a trivial question of what convention you suggest. Recently, I have seen many examples of people writing dict(key1=val1, key2=val2) instead of what I think is the more idiomatic {"key1": val1, "key2": val2}. I think the reason is to avoid using "" for the keys, but I am not sure. Perhaps the dict()-syntax looks closer to other languages?

ain't python great .. there's more than one obvious way to do it — wim, Jun 14 '12 at 10:33
An obvious advantage of the `{}` version is that if you later decide to use some non-string keys you don't have to rewrite the whole line. — James, Jun 14 '12 at 10:38
To be on the safe side you could always combine the two - a sort of 'belt and braces' approach: ``kwargs={"key1": "val1", "key2": "val2"}; d = dict(**kwargs)``. — Blair, Jun 14 '12 at 10:39
@larsmans - its safe in the sense you are guaranteed to have used the idiomatic version ;). That suggestion was made with tongue firmly in cheek. — Blair, Jun 14 '12 at 10:44
@wim That contradict the Zen of Python [There should be one-- and preferably only one --obvious way to do it.](http://www.python.org/dev/peps/pep-0020/) — Kien Truong, Jun 14 '12 at 10:57
@Dikei So the python creators have violated the Zen of Python by making `dict` accept keyword arguments? — Lauritz V. Thaulow, Jun 14 '12 at 11:03
Well, the Zen of Python is a guideline, not a requirement :) — Kien Truong, Jun 14 '12 at 11:25
take a look at : http://stackoverflow.com/questions/6388187/what-is-the-proper-way-to-format-a-multi-line-dict-in-python/6389593#6389593 — dugres, Jun 14 '12 at 11:32

score 8 · Answer 1 · answered Jun 14 '12 at 10:41

{"key1": val1, "key2": val2} is more idiomatic; I hardly ever encounter dict with keyword arguments and I've certainly never been tempted to write it. It's also more general, because keyword arguments have to be Python identifiers:

>>> {"foo bar": 1}
{'foo bar': 1}
>>> dict(foo bar=1)
------------------------------------------------------------
   File "<ipython console>", line 1
     dict(foo bar=1)
                ^
SyntaxError: invalid syntax

>>> dict("foo bar"=1)
------------------------------------------------------------
   File "<ipython console>", line 1
SyntaxError: keyword can't be an expression (<ipython console>, line 1)

There are cases when Python uses words where other languages use punctuations. It produces less line noise. `dict()` might be more readable when it can be used (Obviously it can't be used if keys are not valid Python identifiers; in that case there is no question what to use). — jfs, Jun 14 '12 at 10:49

Lauritz V. Thaulow · Accepted Answer · 2012-06-14T11:09:45.893

5

I'm going to go against the flow here:

Use the dict() method if it suits you, but keep the limitations outlined in other answers in mind. There are some advantages to this method:

It looks less cluttered in some circumstances
It's 3 characters less per key-value pair
On keyboard layouts where { and } are awkward to type (AltGr-7 and AltGr-0 here) it's faster to type out

edited Jun 14 '12 at 11:09

answered Jun 14 '12 at 10:59

Lauritz V. Thaulow

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There is obviously not a correct answer here, but I like yours best. I think SO has an unfortunate tendency to automatically vote up comments by users with the highest reputation. – Gurgeh Jun 15 '12 at 08:27

score 2 · Answer 3 · answered Jun 14 '12 at 10:32

2

Definitely, use {}, keep code simple & type less is always my target

answered Jun 14 '12 at 10:32

zinking

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Well, here you can't have both. For two or more keys, the `dict()` method will mean less typing: two quotes and one space character less per key-value pair, if you follow PEP8 recommendations. – Lauritz V. Thaulow Jun 14 '12 at 10:49

score 1 · Answer 4 · answered Jun 14 '12 at 12:32

1

There is some ambiguity with the braces { }. Consider:

>>> x = {'one','two','three'}
>>> type(x)
<type 'set'>
>>> x = {}
>>> type(x)
<type 'dict'>

Whereas there is no ambiguity with using set() or dict().

answered Jun 14 '12 at 12:32

cdarke

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Good point but this is very minor when you consider the other points being made. – jamylak Jun 14 '12 at 13:10

Should I write dict or {} in Python when constructing a dictionary with string keys?

4 Answers4