Válas Péter wrote:
Hi,
I think I am new to here, as far as I remember. :-)
http://docs.python.org/dev/library/stdtypes.html#dict says:
we can create a dictionary with
- dict({'one': 1, 'two': 2})
What is the adventage of this form to simply writing d = {'one': 1, 'two': 2}?
Is there any difference?
dict() is a function (technically, a type) that creates new dictionaries
from whatever argument you pass to it. { ... } is syntax for creating
literal dictionaries. Think of this as similar to the difference between
a mathematical expression:
x = 2*9-1
and a number literal:
x = 17
HOWEVER, in the above example with dict(), the form shown is redundant.
dict({'one': 1, 'two': 2}) does these four steps:
(a) Python creates a dictionary using the "dict literal" syntax
{'one': 1, 'two': 2}
(b) That dictionary is then passed to the dict() function
(c) The dict() function makes a copy of that dictionary and returns it
(d) Python's garbage collector deletes the original dictionary.
Never put a lone dict literal {...} inside a call to dict(), that's just
a waste of time. Just use the literal on its own.
dict() *can* be very useful, just not in the example shown. You can use
it to make copies of other dicts:
first_dict = {'one': 1, 'two': 2}
second_dict = dict(first_dict)
That's not very interesting, as you can easily make a copy with
first_dict.copy() instead. But it gets more interesting if you want to
add new items to the dictionary:
third_dict = dict(first_dict, three=3, four=4)
You can even leave out the original dict:
fourth_dict = dict(a=1, b=2, c=3)
or instead use a list of (key, value) pairs:
items = [('a', 1), ('b', 2), ('c', 3)]
fifth_dict = dict(items, d=4)
--
Steven
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