Python: Dictionary key name that changes dynamically in a loop

Question

My minimal working example is the following: I have a loop iterating a certain number of times. At each iteration, I would like to create a new key with a name depending on the current index value, for instance key_j, and assign a certain value to it. Is there a way to do this?

for j in range(10):
    dict[key_j] = j**2

Thank you

`key_j = f(j)` => you're done. (f transforms j into something else, string or integer or float or tuple,... hashable) — Jean-François Fabre, Jul 03 '17 at 08:52
@Matthias the point is that "key" is actually a name I really need in the dictionary! — Euler_Salter, Jul 03 '17 at 08:53
`new_dict = {'key_{}'.format(j): j**2 for j in range(10)}` ? Unless you're trying to refer to a variable `key_j` — asongtoruin, Jul 03 '17 at 08:53
But I need this key for different values (the dictionary is much larger) — Euler_Salter, Jul 03 '17 at 08:54
You need to explain the problem in more detail - what do you mean you 'need this key'? Can you give an example of the required dataset? — Jack Parkinson, Jul 03 '17 at 08:55
using `dict` as variable name is not a good practice, it's a type name. — Shai, Jul 03 '17 at 08:59

Shai · Accepted Answer · 2018-05-08T07:31:27.267

You can use string formatting to create a string key with the current loop index

res = {}
for j in xrange(10):
    key_j = 'key_{}'.format(j)  # a string depending on j
    res[key_j] = j**2

The resulting res dictionary is:

{'key_5': 25, 'key_4': 16, 'key_7': 49, 'key_6': 36, 
 'key_1': 1, 'key_0': 0, 'key_3': 9, 'key_2': 4, 'key_9': 81, 
 'key_8': 64}

Note that dictionary keys are not ordered. If you want to keep the order, you need to use OrderedDict instead of regular dict.

BTW,
dictionary keys do not have to be strings, you can use int as keys as well (in fact every "hashable" object can be used as a key):

res = {}
for j in xrange(10):
    res[j] = j**2 # int as key

Resulting with:

{0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49, 8: 64, 9: 81}

In this example the keys are ordered, but it is not guaranteed to be so.

Note that you can create res dictionary using dictionary comprehension, for example:

res = {j: j**2 for j in xrange(10)}

or

res = {'key_{}'.format(j): j**2 for j in xrange(10)}

DexJ · Answer 2 · 2017-07-03T09:06:15.750

2

this will wok simply,

for j in range(10):
    # since your key format is simple you can write in one line
    dict['key_%s'%j] = j**2

Try to rename dict since it is type it is not good practice to use as variable name

edited Jul 03 '17 at 09:06

answered Jul 03 '17 at 09:00

DexJ

1,264
13
24

score 2 · Answer 3 · answered Jan 12 '21 at 13:29

You can use pythons f strings to generate keys based on the index and then use these keys to create dictionary as follows

my_dict = {}
for j in range(4):
  key = f'key_{j}'
  my_dict[key] = j**2
  
print(my_dict)
#{'key_0': 0, 'key_1': 1, 'key_2': 4, 'key_3': 9,}

Python: Dictionary key name that changes dynamically in a loop

3 Answers3