Charles Karl Becker wrote:
I'm trying to use a list comprehension to build a list with a variable
number of lists nested within it (ideally eventually going several
levels of nesting). However I seem to be observing some strange
behavior and was wondering if anyone could take a look at this and
tell me if what I'm trying to do with list comps is possible, or is a
map() or for loop the best thing here?
When in doubt, always use a for loop. List comps can't do anything that
for loops can do, in fact they can do LESS.
I'm not worrying about incrementing the variables in the later
examples since I'm confused about their behavior (just simply adding
new elements to the list rather than nesting the lists; and then
setting the list to [none] in the last uncommented. The last
commented one produces a syntax error, is it impossible to be
recursive with list comps like that or is my syntax just faulty?)
Your syntax is faulty. List comps are not full-blown replacements for
for-loops, they are intentionally simple and straightforward.
board_size = 5
master_list = []
# this block produces the desired behavior
for c in range(board_size):
cell = ['', c+1]
master_list.append(cell)
print(master_list)
# I don't understand why this block behaves the way it does
master_list = []
master_list = [board_size * cell]
print(master_list)
You start of with master_list set to an empty list.
Then you immediately throw away that empty list, and set master_list to
a list containing a single value, board_size * cell. Since by accident
cell happens to equal a list left over from the previous part of code,
you multiply a number 5 by a list ['', 5].
Multiplication of lists performs repetition:
py> ['a', 'b', 'c']*3
['a', 'b', 'c', 'a', 'b', 'c', 'a', 'b', 'c']
By the way, there is no list comprehension there. All you have is a list
containing a single item.
[42] is not a list comp, it is a list containing a single item, 42.
[4*x] is not a list comp, it is a list containing a single item, 4*x
(whatever x happens to be).
[4*x for x in (25, 36, 19, 5)] is a list comp.
# I also don't understand why this block behaves the way that it does
master_list = []
master_list = [master_list.append(cell)]
print(master_list)
You call master_list.append, which modifies master_list in place and
returns None. So you append cell to master_list, then you create a new
list [None], and set master_list to that new list, throwing away the
work you did earlier.
Again, there is no list comprehension here either.
# this block produces a syntax error, and I'm not sure why
'''
master_list = []
master_list = [x for c in range(board_size) master_list.append(cell)]
print(master_list)
'''
Because you don't have a list comprehension. You can't put add arbitrary
code inside a square brackets [ ]. You have to follow the syntax for a
list comprehension:
listcomp = [expression for name in sequence]
not
listcomp = [expression for name in sequence another_command]
--
Steven
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