no side effects
pyth at devel.trillke.net
Wed Jan 8 14:48:03 CET 2003
Michele Simionato wrote:
> I was surprised by the following code:
> >>> for i in [1,2,3]:
> ... print i,
> ... i=3
> I would have expected only 1 to be printed, but instead Python
> continues the loop without noticing that the value of i has
> changed. IOW, no side effect.
> I am not against that (I am even in favor), but it fooled my
> intuition coming from other languages.
> Maybe others will appreciate that.
One of the important things to know about python is the
namespace concept. In a namespace (be it local or global)
a name is bound to an object. In
for i in [1,2,3]:
the for-loop will iteratively bind the (local) name to
1, then 2, then 3. It never looks at the binding of
the name 'i'. With 'i=3' you only change the binding
of 'i' to the object '3' but the for-loop will blindly
change the binding again.
To see the difference, in C, for example, a name points
to a memory location and 'i=3;' will do something to
the memory location.
Python's namespace abstraction is truly powerful although
sometimes surprising if you are used to thinking of
'variables' and 'memory locations'.
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