Appending to list in Python dictionary [duplicate]

by

list.append returns None, since it is an in-place operation and you are assigning it back to dates_dict[key]. So, the next time when you do dates_dict.get(key, []).append you are actually doing None.append. That is why it is failing. Instead, you can simply do

dates_dict.setdefault(key, []).append(date)

But, we have collections.defaultdict for this purpose only. You can do something like this

from collections import defaultdict
dates_dict = defaultdict(list)
for key, date in cur:
    dates_dict[key].append(date)

This will create a new list object, if the key is not found in the dictionary.

Note: Since the defaultdict will create a new list if the key is not found in the dictionary, this will have unintented side-effects. For example, if you simply want to retrieve a value for the key, which is not there, it will create a new list and return it.

More Related Contents:

  1. How to return dictionary keys as a list in Python?
  2. One liner: creating a dictionary from list with indices as keys
  3. How do I return dictionary keys as a list in Python?
  4. Changing one dict changes all dicts in a list of dicts [duplicate]
  5. Python – How to convert only numbers in a mixed list into float?
  6. How to convert only numbers in a mixed list into float
  7. How to name a file using a value in a list in a dictionary?
  8. User input integer list [duplicate]
  9. Replace values in a pandas series via dictionary efficiently
  10. How to merge multiple dicts with same key or different key?
  11. Dictionary: Get list of values for list of keys
  12. Why is a list comprehension so much faster than appending to a list?
  13. Why is dictionary ordering non-deterministic?
  14. Create a list with initial capacity in Python
  15. Error: ” ‘dict’ object has no attribute ‘iteritems’ “
  16. How do I format a string using a dictionary in python-3.x?
  17. python filter list of dictionaries based on key value
  18. Merging dictionary value lists in python
  19. Sorting a dictionary with lists as values, according to an element from the list
  20. Print list of lists in separate lines
  21. Is there anything faster than dict()?
  22. Setting a value in a nested Python dictionary given a list of indices and value
  23. Convert list of strings to dictionary
  24. How to apply a function to each sublist of a list in python?
  25. Creating a dictionary with list of lists in Python
  26. How to sort a list of strings in reverse order without using reverse=True parameter?
  27. Size of a Python list in memory
  28. How to extract all values from a dictionary in Python?
  29. Comparing 2 lists consisting of dictionaries with unique keys in python
  30. TypedDict when keys have invalid names

Leave a Comment