Yes, loops don't have their own scope. Indeed, very few flow elements in python -- if, with, try/except -- create a new scope. In that sense, it's fairly consistent, but can be unexpected for people that have used languages with many nested scopes.
The lambda behavior is a common gotcha - there are hundreds of questions on StackOverflow that are caused by that misunderstanding http://stackoverflow.com/questions/7368522/weird-behavior-lambda-inside-list-comprehension The most common solution is to just provide the variable as a default argument to the lambda function, which will bind it how you'd expect [lambda base, exponent=exponent: (base ** exponent) for exponent in range(9)] On Thu, Sep 29, 2016 at 12:29 PM, <namenobodywa...@gmail.com> wrote: > hello pythonistas > > i've had a nodding acquaintance with python for some time, and all along i > assumed that for-loops got a namespace of their own; now i'm reading up on > the language and i find out that's not the case: the loop variable gets put > into the enclosing namespace, overwriting any former value that was already > there; of course i realize there are situations where one is interested in > the value that a loop variable has after the loop has been exited, but this > behavior seems grossly unintuitive to me: has there ever been any > discussion of giving for-loops the option of running in namespaces of their > own? > > and it gets even worse; consider the following means of raising an > exception: > > #)-------------------------------------- begin code > > def powerfunction(exponent): > return lambda base: (base ** exponent) > > p1 = [powerfunction(exponent) for exponent in range(9)] > p2 = [lambda base: (base ** exponent) for exponent in range(9)] > > assert p1[3](4) == p2[3](4) > > #)---------------------------------------- end code > > apparently the problem is that "exponent" gets evaluated when the relevant > lambda function is run, not when it's defined, but no binding for > "exponent" gets put into the enclosing namespace: it seems that python > remembers this ghostly "exponent" for me on the theory that i don't WANT > "p1" and "p2" to be the same; can anyone help to reconcile me to this > semantics? > > thanks if you can help > stm > > > > > > > > > > -- > https://mail.python.org/mailman/listinfo/python-list > -- https://mail.python.org/mailman/listinfo/python-list
