I am using Python's collections library to make a dictionary where the keys are integers and the values are lists. I am using the defaultdict(list) command. I am trying, but unsuccessfully to edit the elements in these lists which are values.
I thought list comprehension should work for this but I keep getting syntax errors. I attach what I have tried below:
import collections
test = collections.defaultdict(list)
test[4].append(1)
test[4].append(5)
test[4].append(6)
#This would yield {4: [1,5,6]}
run_lengths = [1,3,4,6] #dummy data
for i in run_lengths:
#I would like to add 3 to each element of these lists which are values.
test[i][j for j in test[i]] += i
Assuming you want to modify the list in place, you need to overwrite each element as integers are immutable:
test[4][:] = [e+3 for e in test[4]]
Output:
defaultdict(list, {4: [4, 8, 9]})
If you don't care about generating a new object (i.e. you did not link a variable name to test[4] you can just use:
test[4] = [e+3 for e in test[4]]
The first case modifies the list in place. If other variables point to the list, the changes will be reflected:
x = test[4]
test[4][:] = [e+3 for e in test[4]]
print(x, test)
# [4, 8, 9] defaultdict(<class 'list'>, {4: [4, 8, 9]})
In the other case, the list is replaced by a new, independent, one,. All potential bindings are lost:
x = test[4]
test[4] = [e+3 for e in test[4]]
print(x, test)
# [1, 5, 6] defaultdict(<class 'list'>, {4: [4, 8, 9]})
Assuming run_lengths contains a list of keys to update:
for i in run_lengths:
test[i][:] = [e+3 for e in test[i]]