我如何在python中浏览（x，y）列表列表

Question

How can I walk through a list of list of (x,y) in python?

I have a data structure like this in python, which is a list of a list of a (x,y)

coords = [
      [[490, 185] , [490, 254], [490, 312] ],  # 0
      [[420, 135] , [492, 234], [491, 313], [325, 352] ],  # 1
]

I want to walk through the list and get x, y of each set:

# where count goes from 0 to 1
 a_set_coord[] = coords[count]
 for (tx, ty) in a_set_coord:
    print "tx = " + tx + " ty = " + ty

But I get error:

SyntaxError: ("no viable alternative at input ']'"

How can I fix this?

Answer 1

Remove the brackets after a_set_coord :

a_set_coord = coords[count]

Also, the print statement tries to concatenate strings and ints. Change it to:

print "tx = %d ty = %d" % (tx, ty)

Answer 2

If you just want to flatten your list of lists by one level, itertools.chain or itertools.chain.from_iterable could be very helpful:

>>> coords = [
...       [[490, 185] , [490, 254], [490, 312] ],  # 0
...       [[420, 135] , [492, 234], [491, 313], [325, 352] ],  # 1
... ]
>>> import itertools as it
>>> for x,y in it.chain.from_iterable(coords):
...     print ('tx = {0} ty = {1}'.format(x,y))
... 
tx = 490 ty = 185
tx = 490 ty = 254
tx = 490 ty = 312
tx = 420 ty = 135
tx = 492 ty = 234
tx = 491 ty = 313
tx = 325 ty = 352

Answer 3

Use a simple for loop.

for i in coords:
   x = i[0]
   y = i[1]
   if len(i) == 3: z = i[2] # if there is a 'z' coordinate for a 3D graph.
   print(x, y, z)

This assumes that each list in coords is only of length 2 or 3. If it is different, this will not work. However, considering the lists are coordinates, it should be fine.