---
id: practical-python-2.17
source_exercise_id: "2.17"
title: "Inverting a dictionary"
section: "2.4 Sequences"
source_path: "02_Working_with_data/04_Sequences.md"
source_repo: "https://github.com/dabeaz-course/practical-python"
source_commit: "93dca856b41c61a0a0f85ae334116e4c125629ea"
student_visible_solution: false
has_private_solution: false
skip: false
---

# Exercise 2.17: Inverting a dictionary

> Source: Practical Python Programming, `02_Working_with_data/04_Sequences.md`.

### Exercise 2.17: Inverting a dictionary

A dictionary maps keys to values. For example, a dictionary of stock prices.

```python
>>> prices = {
        'GOOG' : 490.1,
        'AA' : 23.45,
        'IBM' : 91.1,
        'MSFT' : 34.23
    }
>>>
```

If you use the `items()` method, you can get `(key,value)` pairs:

```python
>>> prices.items()
dict_items([('GOOG', 490.1), ('AA', 23.45), ('IBM', 91.1), ('MSFT', 34.23)])
>>>
```

However, what if you wanted to get a list of `(value, key)` pairs instead?
*Hint: use `zip()`.*

```python
>>> pricelist = list(zip(prices.values(),prices.keys()))
>>> pricelist
[(490.1, 'GOOG'), (23.45, 'AA'), (91.1, 'IBM'), (34.23, 'MSFT')]
>>>
```

Why would you do this? For one, it allows you to perform certain kinds
of data processing on the dictionary data.

```python
>>> min(pricelist)
(23.45, 'AA')
>>> max(pricelist)
(490.1, 'GOOG')
>>> sorted(pricelist)
[(23.45, 'AA'), (34.23, 'MSFT'), (91.1, 'IBM'), (490.1, 'GOOG')]
>>>
```

This also illustrates an important feature of tuples. When used in
comparisons, tuples are compared element-by-element starting with the
first item. Similar to how strings are compared
character-by-character.

`zip()` is often used in situations like this where you need to pair
up data from different places.  For example, pairing up the column
names with column values in order to make a dictionary of named
values.

Note that `zip()` is not limited to pairs. For example, you can use it
with any number of input lists:

```python
>>> a = [1, 2, 3, 4]
>>> b = ['w', 'x', 'y', 'z']
>>> c = [0.2, 0.4, 0.6, 0.8]
>>> list(zip(a, b, c))
[(1, 'w', 0.2), (2, 'x', 0.4), (3, 'y', 0.6), (4, 'z', 0.8))]
>>>
```

Also, be aware that `zip()` stops once the shortest input sequence is exhausted.

```python
>>> a = [1, 2, 3, 4, 5, 6]
>>> b = ['x', 'y', 'z']
>>> list(zip(a,b))
[(1, 'x'), (2, 'y'), (3, 'z')]
>>>
```

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## 关联来源

- [[summaries/04_Sequences]]