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?

Answer 1

{"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)

Answer 2

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

Answer 3

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

Answer 4

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().