How to start from second key when iterating over dictionary using for loop in Python

Question

I am computing returns from data in a dictionary. My keys are dates and for every key I have a dataframe with data to compute my returns. To compute the returns I need data today and yesterday (t and t-1), hence I want to initiate from the second observation (key). Since I do not have much experience my initial thought was to execute like this:

dict_return = {}
for t, value in dict_data.items()[1:]:
   returns = 'formula'
   dict_returns[t] = returns

Which gave me the error:

TypeError: 'dict_items' object is not subscriptable

Searching for an answer, the only discussion I could find was skipping the first item, e.g. like this:

from itertools import islice

for key, value in islice(largeSet.items(), 1, None): 

Is there a simple approach to skip the first key?

Thank you

Answer 1

If you are in Python 3 you need to use a list, Dict_ items ([‘No surfacing ‘,’flippers’]) returns a dict_ The items object is no longer of the list type and does not support index, this is why the list type can be used

Answer 2

I can think of 2 options, both require an extra step:

Option 1: Create a second dict without your key and loop over that

loop_dict = dict_data.pop(<key_to_remove>)

Then loop over loop_dict as you have done above.

Option 2: Create a list of keys from your dict and loop over that

keys = dict_data.keys()
loop_keys = keys[1:]

for key in loop_keys:
    Etc

Answer 3

If you pass a reference to your dictionary to list() you will get a list of the dictionary's keys. This is because dictionaries are iterable. In your code you're not interested in the key's value so:

dict_data = {'a': 1, 'b': 2} # or whatever

dict_data[list(dict_data)[1]] = 3

print(dict_data)

Output:

{'a': 1, 'b': 3}