12.4. Iterating Through Dictionaries

Many times in this book, we looped through the characters in a string or the elements in a list without using the index values.

Example

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my_string = 'Rutabagas!'
numbers = [33, -25, 3.14, 86, 1168, 42, 6.022e23]

for character in my_string:
   print(character)

total = 0
for number in numbers:
   total += number*2

Each time the loop body repeats, the loop variable (character or number) gets assigned the next value in the collection.

We can do something similar with dictionaries.

12.4.1. Loop by Keys or Values

To loop through a dictionary, we need to specify whether to assign the loop variable the keys or the values from the collection. To do this, we use the keys() or values() method in the for statement:

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for key in dictionary_name.keys():
   # Loop body...

for value in dictionary_name.values():
   # Loop body...

In the first for loop, each time the code repeats, the loop variable key gets assigned one of the key names from the dictionary. In the second loop, the variable value gets assigned a new value each iteration.

Note

We do not have to use the name key or value for the loop variable, but doing so helps keep our code clear.

Try It!

Run the following program to see how iterating through a dictionary works.

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comics = {
   'Gary Larson' : 'The Far Side',
   'Terri Libenson' : 'Pajama Diaries',
   'Hilary B. Price' : 'Rhymes with Orange',
   'Jim Toomey' : "Sherman's Lagoon"
}

# Iterate by keys.
print("Here are the keys from the comics dictionary:")
for key in comics.keys():
   print(key)

# Iterate by values.
print("\nHere are the values from the comics dictionary:")
for value in comics.values():
   print(value)

# Access both keys and values.
print("\nHere are the key/value pairs from the comics dictionary:")
for key in comics.keys():
   print(f"Key: {key}, Value: {comics[key]}")

# Change the dictionary values.
print("\nUse a loop to change all of the values:")
for key in comics.keys():
   comics[key] = comics[key].upper()

print(comics)

repl.it

Try adding or removing key/value pairs to change the output.

Note that on lines 21 and 26, we access each value in the dictionary by using bracket notation and the loop variable key. Each iteration, key represents a new label from comics, so comics[key] accesses a different value each time the loop repeats.

Try It!

What happens if we forget to attach the keys() or values() method in the for statement? Let’s find out!

Run the following program to see how Python deals with a missing method.

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comics = {
   'Gary Larson' : 'The Far Side',
   'Terri Libenson' : 'Pajama Diaries',
   'Hilary B. Price' : 'Rhymes with Orange',
   'Jim Toomey' : "Sherman's Lagoon"
}

# What gets printed if we don't attach .keys() or .values() to comics?
for data in comics:
   print(data)

repl.it

Aha! By default, Python iterates through a dictionary by using the key names.

12.4.1.1. Loop by Key/Value Pairs

The items() method returns each key/value pair as a unit, and it allows us to assign BOTH the key and value from the dictionary to separate variables. The general syntax for this is:

for (key, value) in dictionary_name.items():

In the for statement, we define a pair of variables (key, value) to hold a key name and its linked value from the dictionary. Each iteration, these two variables represent a new key/value pair from the collection.

Example

Compare the following two loops, which do exactly the same thing:

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comics = {
   'Gary Larson' : 'The Far Side',
   'Terri Libenson' : 'Pajama Diaries',
   'Hilary B. Price' : 'Rhymes with Orange',
   'Jim Toomey' : "Sherman's Lagoon"
}

# Iterate by keys, and print out the dictionary key/value pairs:
for key in comics.keys():
   print(key, comics[key])

# Iterate by key/value pairs:
for (key, value) in comics.items():
   print(key, value)

By defining a pair of variables, we can access the values from the dictionary without needing to use bracket notation. On line 14, the variable value replaces comics[key] in our code.

12.4.2. Sorting by Keys

Dictionaries are unordered collections, so they do NOT include any type of sorting method. However, sometimes we might want to access or display the key/value pairs in a particular order—like alphabetically by key name.

If we want to sort a dictionary, the short answer is…we can’t. However, we can use a work-around. We won’t change the order of the key/value pairs in the dictionary. Instead, we will use a list, which can be sorted.

Try It!

Let’s see how to print out the key/value pairs in a dictionary alphabetically by key name.

  1. Run the program as-is. Notice that the loop prints the key/value pairs in the order they occur in the collection.

  2. On line 13, add the statement keys_list = list(grocery_bill.keys()). On line 14, print keys_list and run the program to see the result.

    • Line 13 does NOT change the dictionary. Instead, the list() function creates a new list that contains copies of all the key names from grocery_bill.

  3. On line 14, replace the print statement with keys_list.sort(), which alphabetizes the strings in the list.

    • Note that line 14 sorts the list and NOT the dictionary!

  4. Change the for statement to loop through keys_list instead of grocery_bill.keys(). Run the program again so see the alphabetical result.

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#!/bin/python3
grocery_bill = {
   'bananas' : 1.77,
   'pears' : 6.97,
   'broccoli' : 1.98,
   'cheese' : 5.99,
   'soy milk' : 6.99,
   'orange juice' : 5.49,
   'eggs' : 1.98,
   'carrots' : 2.99
}

for key in grocery_bill.keys():
   print("{0}: {1}".format(key, grocery_bill[key]))

repl.it

Note that we do not need to change the final print statement. It still uses the loop variable key, but in this case it takes elements from the list we created in line 13 and sorted in line 14.

grocery_bill[key] still refers to a value from the dictionary, but now we access the values in a different (sorted) order!

If we print the grocery_bill dictionary at the end of the program, we can prove that we did NOT alter the order of the key/value paris.

By adding copies of the key names to a separate list, we can sort the list to get the order we want. Then we use the key names from the sorted list to access the values in the dictionary.

12.4.3. Check Your Understanding

Question

Given the code:

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comics = {
   'Georgia Dunn' : 'Breaking Cat News',
   'Jan Eliot' : 'Stone Soup',
   'Wiley Miller' : 'Non Sequitur',
   'Bill Watterson' : 'Calvin and Hobbs'
}

for key in comics.keys():
   print(key, comics[key])

What is the value of comics[key] the third time through the loop?

  1. 'Wiley Miller'

  2. 'Bill Watterson'

  3. 'Non Sequitur'

  4. 'Calvin and Hobbs'