Python is Considered Harmful

[Alex Martelli]

Isaac To wrote:
   ...
> Did you really mean
> 
>   map(lambda f: f(1), [lambda x: x+i for i in range(3)])
> 
> ?  If so, it does return [3, 3, 3].  The only problem here is that Python
> variables are name lookups over dictionaries, and as such the value of "i"
> is not coined into the lambda.  Rather it is "external", binded
> dynamically. I believe the behaviour can be fixed rather easily by some
> directives, say making it
> 
>   map(lambda f: f(1), [lambda x: x+(*i) for i in range(3)])
> 
> where the * construct would get the reference of i at function definition
> time rather than at function invocation time (anyone can point me to an
> PEP?).  On the other hand, I basically never use lambda this way.

No new "directive" needed: it's easy to get "snap-shots" of current
values of names into a lambda or other function:

map(lambda f: f(1), [lambda x, i=i: x+i for i in range(3)])

The "i=i" gets the (outer) value of i at function-definition time and
injects it as local variable i in the lambda.  You can also rename just
as easily (and sometimes it's clearer):

map(lambda f: f(1), [lambda x, k=i: x+k for i in range(3)])



Alex