When to clear a dictionary...

Gabriel Genellina gagsl-py2 at yahoo.com.ar
Fri Apr 20 23:07:40 CEST 2007

En Fri, 20 Apr 2007 14:28:00 -0300, Larry Bates <larry.bates at websafe.com>  

> Bill Jackson wrote:
>> What is the benefit of clearing a dictionary, when you can just reassign
>> it as empty?
> If you have objects that point to the dictionary (something like a cache)
> then you want to clear the existing dictionary instead of just assigning
> it to empty.  If nothing points to it, assigning it to empty is fast and
> you can let garbage collection do the rest.

For an actual comparision, see Alex Martelli posts a few days ago:

>>>>> a = {1:2,3:4}
>>>>> b = {1:2:4:3}
>>>>> a.clear()
>>>>> a.update(b)
>>>>> a = {1:2,3:4}
>>>>> b = {1:2,4:3}
>>>>> for key in b:
>> ...     a[key] = b[key]

> Syntax error in the first example but if you fix that the first two are
> equivalent (but I would suspect that the second would be faster for large
> dictionaries).

It's the other way; the first method contains a single Python function  
call and most of the work is done in C code; the second does the iteration  
in Python code and is about 4x slower.

> python -m timeit -s "b=dict.fromkeys(range(10000));a={}" "a.update(b)"
100 loops, best of 3: 10.2 msec per loop

> python -m timeit -s "b=dict.fromkeys(range(10000));a={}" "for key in b:  
> a[key]=b[key]"
10 loops, best of 3: 39.6 msec per loop

Gabriel Genellina

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