Defining and appending in new list inside a loop using iterator (python)

Question

I am trying to start and append new lists inside a loop using iterator. I tried to use dictionary idea from this answer.

Here is the snippet of my code, you can ignore the details of the code. If the if condition is satisfied I need to append values of i in a specific list, which will be like patch12,patch53 etc:

import math as m

d = {}

for i in range(0,10):

     for j in range(0, 10):

          if((m.floor(vector[upper_leaflet[i],0:1]) <= xx[j][0]) & (m.floor(vector[upper_leaflet[i],0:1]) < yy[j][0])):

              d["patch{}{}".format(xx[j][0],yy[j][0])].append(i)

Output should be something like if I print patch52 = [ 1, 5, 9 ,10] What would be the correct way to do perform this?

Could you expand on *"doesn't work"*? You seem to be trying to append to a string, which obviously isn't going to work - what was the expected output? — jonrsharpe, Dec 23 '16 at 11:13

score 2 · Accepted Answer · edited May 23 '17 at 12:24

You are trying to create dynamic variables with name patch12 or patch53 etc. This can not be done just by formatting a string. Look at this thread where dynamic variable creation is explained.

However, its not a good programming practice. Correct solution could be to use a dictionary or a list. example:

patches = {}
for i in range(0,10):

     for j in range(0, 10):

          if((m.floor(vector[upper_leaflet[i],0:1]) <= xx[j][0]) & (m.floor(vector[upper_leaflet[i],0:1]) < yy[j][0])):

              key_name = "patch{}{}".format(xx[j][0],yy[j][0])
              #if key already exists, append to that list, if not create new list
              if key_name in patches:
                    key_value = patches[key_name]
              else:
                    key_value = []
              #appending newly value ..
              key_value.append(i)
              patches[key_name] = key_value

Further , in your program when you need to use this data, either your can iterate through dictionary to find out all keys ex:

for patch in patches:
    do something...

or you can directly access value of a particular key (ex: patch52 by

value = patches['patch52']

which will give you a list of values corresponding to patch52 ex:

print(patches['patch52'])
[1,4,7,9]

Oh and yes: this code is broken, `list.append()` returns `None` — bruno desthuilliers, Dec 23 '16 at 11:41

score 0 · Answer 2 · answered Dec 23 '16 at 11:43

Using a collections.defaultdict makes things easier:

import math as m
from collections import defaultdict

d = defaultdict(list)
for i in range(0,10):
     for j in range(0, 10):
          if ((m.floor(vector[upper_leaflet[i],0:1]) <= xx[j][0]) & (m.floor(vector[upper_leaflet[i],0:1]) < yy[j][0])):
              d["patch{}{}".format(xx[j][0],yy[j][0])].append(i)

Defining and appending in new list inside a loop using iterator (python)

2 Answers2