Deepcopying a byte string is quicker than copying it - problem?

[Florian Leitner]

On 27.02.14, 7:30 , Frank Millman wrote:
> Hi all
> 
> I noticed this a little while ago, but dismissed it as a curiosity.
> On reflection, I decided to mention it here in case it indicates a
> problem.
> 
> This is with python 3.3.2.
> 
> C:\>python -m timeit -s "import copy" "copy.copy('a'*1000)" 100000
> loops, best of 3: 6.91 usec per loop
> 
> C:\>python -m timeit -s "import copy" "copy.deepcopy('a'*1000)" 
> 100000 loops, best of 3: 11.8 usec per loop
> 
> C:\>python -m timeit -s "import copy" "copy.copy(b'a'*1000)" 10000
> loops, best of 3: 79.9 usec per loop
> 
> C:\>python -m timeit -s "import copy" "copy.deepcopy(b'a'*1000)" 
> 100000 loops, best of 3: 11.7 usec per loop
> 
> As you can see, deepcopying a string is slightly slower than
> copying it.
> 
> However, deepcopying a byte string is orders of magnitude quicker
> than copying it.
> 
> Actually, looking closer, it is the 'copy' that is slow, not the
'deepcopy'
> that is quick..
> 
> Expected, or odd?
> 
> Frank Millman
> 
> 
> 

Indeed, just tried this:

Python 3.3.2 (default, Sep 30 2013, 21:37:29)
[GCC 4.2.1 Compatible Apple LLVM 5.0 (clang-500.2.75)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import copy
>>> 
>>> bs = b'a'*1000 id(bs)
140246553640960
>>> id(copy.copy(bs)) # WATCH THIS!
140246578636800
>>> id(copy.deepcopy(bs))
140246553640960
>>> id(bs[:])
140246553640960
>>> 
>>> us = 'a'*1000 id(us)
140246553642496
>>> id(copy.copy(us))
140246553642496
>>> id(copy.deepcopy(us))
140246553642496
>>> id(us[:])
140246553642496

Interesting; copy.copy(b'bytes') creates a "real" copy of an otherwise
immutable value... That might actually be some form of a bug.

By the way, if you are looking into this as something about speed, you
should use slices, not copy.copy/deepcopy, anyways (copy = bc[:]);
More than 5x faster:

$ python -m timeit -s "import copy; s='a'*1000" "copy.copy(s)"
1000000 loops, best of 3: 0.465 usec per loop
$ python -m timeit -s "import copy; s='a'*1000" "cs = s[:]"
10000000 loops, best of 3: 0.0728 usec per loop

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