dicts

Overview

Teaching: 12 min
Exercises: 8 min

Questions

• What is a Python dict?

• How do I use a dict?

Objectives

• Understand what a dict is.

• Be able to create, modify, and use dicts in Python.

In the previous episode we saw that some APIs will return data formatted as JSON, including names (or keys) and values associated with them.

Since we would ultimately like to work with data from these APIs in Python, it would be nice if Python had a data structure that behaved similarly. In the Software Carpentry introduction to Python, we learned about lists, which are ordered collections of things, indexed by their position in the ordering. What we would like here is similarly a collection, but rather than having ordering and indexing by position, instead we would like elements to have an arbitrary index of our choice.

In fact, Python has such a collection built into it; it is called a dict (short for dictionary). Let’s construct one now, to hold data from the Mayo Clinic about caffeine levels in various beverages.

caffeine_mg_per_serving = {'coffee': 96, 'tea': 47, 'cola': 24, 'energy drink': 29}

We see here that the dict is created within curly braces {}, and contains keys and corresponding values separated by a :, with successive pairs being separated by a , like in a list.

Again, similarly to a list, we can access elements of the dict with square brackets []. For example, to get the number of mg of caffeine per serving of coffee, we could use the following:

print("Coffee has", caffeine_mg_per_serving['coffee'], "mg of caffeine per serving")

Coffee has 96 mg of caffeine per serving

We can also replace elements in the same way that we can for a list. For instance, you may have spotted that the value for 'cola' is incorrect. Let’s fix that now.

caffeine_mg_per_serving['cola'] = 22
print(caffeine_mg_per_serving)

{'coffee': 96, 'tea': 47, 'cola': 22, 'energy drink': 29}

One thing that we can’t do for lists is create new elements by indexing with []. But dicts let us do that, as well:

caffeine_mg_per_serving['green tea'] = 28
print(caffeine_mg_per_serving)

{'coffee': 96, 'tea': 47, 'cola': 22, 'energy drink': 29, 'green tea': 28'}

Ordering

Python dicts historically were not ordered—you would not be guaranteed to get back results in the same order that you put them in. In more recent versions of Python, dicts do preserve the ordering in which they are created, so 'green tea', having been added most recently, appears at the end.

Missing values

dicts will throw an error, though, if we try to access values for keys that we have not added previously.

print(caffeine_mg_per_serving['guarana'])

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-2-dff37d2ef7d1> in <module>
----> 1 caffeine_mg_per_serving['guarana']

KeyError: 'guarana'

To write more robust code, we might like to check whether we can use a particular key before trying to access it. In a list, this is simple, as we can check whether a particular index is less than the length of the list. With a dict, we need to use a keyword to check whether a particular key is in the list:

'coffee' in caffeine_mg_per_serving

True

Alternatively, if we want to get an element of the list and use a default value if the key isn’t found, we can use the .get() method:

print(caffeine_mg_per_serving.get("coffee", 0))
print(caffeine_mg_per_serving.get("hot chocolate", 0))

96
0

(If you don’t specify the default value, then Python uses None for keys that are not found.)

Looping

Now, a particularly useful thing to do with a list is to loop over it. What happens when we loop over a dict?

for item in caffeine_mg_per_serving:
    print(item)

coffee
tea
cola
energy drink
green tea

Looping (or otherwise iterating) over a dict in fact loops over its keys. This matches with what the in keyword does—it would be strange for the two to look at different aspects of the dict. But sometimes we may want to use the values as well as the keys in a loop. We could index back into the dict via the key, but that is repetitive. We can instead use the .items() method of the dict:

for drink, quantity in caffeine_mg_per_serving.items():
    print(drink.capitalize(), "contains", quantity, "mg of caffeine per serving")

Coffee contains 96 mg of caffeine per serving
Tea contains 47 mg of caffeine per serving
Cola contains 22 mg of caffeine per serving
Energy drink contains 29 mg of caffeine per serving
Green tea contains 28 mg of caffeine per serving

What’s in a key?

In this episode, we have used strings as keys, as this is what we’re most likely to see when working with JSON. This is not a Python restriction, however. We can use any “hashable” type as a dict key; this includes strings, numbers, and tuples, among other immutable types. Most notably, this excludes lists and dicts (which are mutable).

dicts of functions

What will the following code do?

import numpy as np

operations = {
    'min': np.min,
    'max': np.max
}

def process(array, operation):
    return operations[operation](array)

print(process([1, 4, 7, 2, -3], 'min'))

When might this kind of behaviour be useful?

Try adjusting the example so that 'mean' and 'std' also work as you might expect.

Solution

This will pull the described function out of the dictionary. This could be useful when you want to allow the user to decide what functionality is desired at run-time, perhaps in a configuration file. Perhaps a choice of inversion algorithms or fitting functions could be offered.

To add other functions, the operations dict could be adjusted as:

operations = {
    'min': np.min,
    'max': np.max,
    'mean': np.mean,
    'std': np.std
}

Nested dicts

It is worth noting that the values in a dict can be of any type (this is not true for the keys). One notable case, is that values can be themselves dicts:

nutrition_values = {'energy': {'units': 'kCal/100g',
                               'values': {'white bread': 273,
                                          'almonds': 512}},
                    'caffeine': {'units': 'mg per serving',
                                 'values': caffeine_mg_per_serving}}

It is then possible to access data using multiple square bracket expressions:

print("Caffeine content of coffee:", nutrition_values['caffeine']['values']['coffee'])
print("Units:", nutrition_values['caffeine']['units'])

Caffeine content of coffee: 96 
Units: mg per serving

Key Points

• A dict is a collection of key-value pairs.

• Create a dict with the syntax {key1: value1, key2: value2, ...}.

• Get and set elements of a dict with square brackets: my_dict[key1] = new_value1.