Python Weird Behavior In List Comprehension

def nrooks(n): #make board print n # prints 4 arr = [0 for n in range(n)] # if 0 for n becomes 0 for x, it works fine print n # prints 3 instead of 4 nrooks(4) Ho

Solution 1:

Python 2

The n variable used in the list comprehension is the same n as is passed in.

The comprehension sets it to 1, 2, and then finally 3.

Instead, change it to

arr = [0 for _ in range(n)]

or (surprisingly!)

arr = list(0 for n in range(n))

Python 3

This has been fixed.

From the BDFL himself:

We also made another change in Python 3, to improve equivalence between list comprehensions and generator expressions. In Python 2, the list comprehension "leaks" the loop control variable into the surrounding scope:

x = 'before'
a = [x for x in1, 2, 3]
print x # this prints '3', not 'before'

This was an artifact of the original implementation of list comprehensions; it was one of Python's "dirty little secrets" for years. It started out as an intentional compromise to make list comprehensions blindingly fast, and while it was not a common pitfall for beginners, it definitely stung people occasionally. For generator expressions we could not do this. Generator expressions are implemented using generators, whose execution requires a separate execution frame...

However, in Python 3, we decided to fix the "dirty little secret" of list comprehensions by using the same implementation strategy as for generator expressions. Thus, in Python 3, the above example (after modification to use print(x) :-) will print 'before'.