Patches item #1391204, was opened at 2005-12-27 08:00
Message generated for change (Comment added) made by jimjjewett
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Category: Core (C code)
Group: Python 2.5
Status: Closed
Resolution: Rejected
Priority: 5
Submitted By: Nicolas Lehuen (nlehuen)
Assigned to: Guido van Rossum (gvanrossum)
Summary: dict.merge
Initial Comment:
If you want to update a dictionary with another one,
you can simply use update :
a = dict(a=1,c=3)
b = dict(a=0,b=2)
a.update(b)
assert a == dict(a=0,b=2,c=3)
However, sometimes you want to merge the second dict
into the first, all while keeping the values that are
already defined in the first. This is useful if you
want to insert default values in the dictionary without
overriding what is already defined.
Currently this can be done in a few different ways, but
all are awkward and/or inefficient :
a = dict(a=1,c=3)
b = dict(a=0,b=2)
Method 1:
for k in b:
if k not in a:
a[k] = b[k]
Method 2:
temp = dict(b)
temp.update(a)
a = temp
This patch adds a merge() method to the dict object,
with the same signature and usage as the update()
method. Under the hood, it simply uses PyDict_Merge()
with the override parameter set to 0 instead of 1.
There's nothing new, therefore : the C API already
provides this functionality (though it is not used in
the dictobject.c scope), so why not expose it ? The
result is :
a = dict(a=1,c=3)
b = dict(a=0,b=2)
a.merge(b)
assert a == dict(a=1,b=2,c=3)
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Comment By: Jim Jewett (jimjjewett)
Date: 2006-01-13 16:52
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The times I have wanted it are for configuration settings.
If I don't find the answer in the first place, I look in the
defaults -- and I would prefer to update the current
settings all at once, since I'm going to the trouble of
creating creating/finding the defaults dictionary. I don't
want to change the defaults, since they typically apply over
a wider scope than the current context.
Note that this isn't a random permutation -- it is an
existing permutation that is already exported to C
extensions, and just not to python code.
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Comment By: Guido van Rossum (gvanrossum)
Date: 2006-01-01 12:55
Message:
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I don't think we need to provide every possible permutation
of existing operations.
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Comment By: Raymond Hettinger (rhettinger)
Date: 2006-01-01 01:17
Message:
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The example is only slightly complicated by the implicit
requirement to keep b unchanged; otherwise, it is easy
enough to swap the dicts before updating: a,b=b,a;
a.update(b).
Anyway, I do not think the use case is sufficiently common
or cumbersome to warrant making the mapping API more complex.
Guido?
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