Dictionary Functions

Earlier, we used dictionary method items to iterate through a dictionary’s key–value pairs. Similarly, built-in functions keys and values can be used to iterate through only a dictionary’s keys or values, respectively:

months = {'January': 1, 'February': 2, 'March': 3}
for month_name in months.keys():
  print(month_name)

for month_num in months.values():
  print(month_num)

Dictionary functions items, keys and values each return a view of a dictionary’s data. When you iterate over a view, it “sees” the dictionary’s current contents—it does not have its own copy of the data.

To show that views do not maintain their own copies of a dictionary’s data, let’s first save the view returned by keys into the variable months_view, then loop through it:

months = {'January': 1, 'February': 2, 'March': 3}
month_views = months.keys()
for key in month_views:
  print(key)

Next, let’s add a new key–value pair to months and display the updated dictionary:

months['December'] = 12

Now, let’s loop through months_view again. The key we added above will be displayed:

for key in month_views:
  print(key) # December will now be displayed

Do not modify a dictionary while iterating through a view. According to the Python Standard Library documentation, either you’ll get a RuntimeError or the loop might not process all of the view’s values.

Converting Dictionaries to Lists:

You might occasionally need lists of a dictionary’s keys, values or key–value pairs. To obtain such a list, pass the view returned by keys, values or items to the built-in `list` function. Modifying these lists does not modify the corresponding dictionary: 
listOfKeys = list(months.keys())
listOfValues = list(months.values())
listOfKeysValues = list(months.items())

Updating Dictionaries:

You may insert and update key–value pairs using dictionary function `update`. First, let’s create an empty country_codes dictionary: 
country_codes = {}

The following update call receives a dictionary of key–value pairs to insert or update:

country_codes.update({'South Africa': 'za'})
Dictionary Operations