Iterating Over Dictionary Keys

Why Process Both Parts

When you iterate through a dictionary, you may need both the key and its corresponding value. For example, a report may need to show each name with its score, or a calculation may need both pieces of information. The items() method provides both at the same time, so you do not need to iterate over keys and look up each value separately.

The Loop Header

for key, val in d.items():
    print(val, key)

This loop header combines three ideas. The for loop visits each tuple returned by d.items(). The tuple assignment key, val unpacks the tuple into two variables. The first element goes into key, and the second element goes into val. On each iteration, both variables are updated to the next key-value pair.

Tracing Each Pair

d = {'a': 10, 'b': 1, 'c': 22} for key, val in d.items(): print(val, key)

The print statement uses val before key, so each output line shows the value first and the key second. The variables do not keep their first assignments; they are updated on every iteration.

From Entry to Output

Each tuple follows the same path. First, items() makes the key-value pair available to the loop. Next, tuple assignment places the first tuple element in key and the second in val. Finally, the loop body uses those variables. In the source example, print(val, key) reverses the display order, but the assignments themselves remain key first and value second.

Order and Unpacking Mistakes

• Assuming the loop is alphabetical or insertion ordered

  The source describes dictionary iteration here as occurring in hash key order. That order depends on how the implementation stores keys internally.

  Fix: When tracing this pattern, use the stated hash key order rather than assuming alphabetical or insertion order.

• Assigning the value to key and the key to val

  Each tuple places the key first and the corresponding value second.

  Fix: Read the assignment from left to right: key receives the first tuple element, and val receives the second.

• Using the wrong number of variables

  Each tuple returned by items() contains exactly one key-value pair, so it has two elements.

  Fix: Use two variables, such as key and val, to match the two-element tuple.

• Predicting output from variable names instead of the print statement

  The output order is controlled by the arguments in the print statement.

  Fix: Trace the assignments first, then read the print statement in its actual order.

Predict Before Running

Pattern Summary

1. The items() method returns a sequence of two-element tuples, with one key-value pair in each tuple.
2. In for key, val in d.items():, tuple assignment puts the key in key and the value in val.
3. The loop updates key and val for every iteration until all tuples have been processed.
4. The print statement determines the order in which the key and value appear in the output.
5. For this topic, trace dictionary entries in hash key order rather than assuming alphabetical or insertion order.