Dictionaries

Last updated on 2025-04-15 | Edit this page

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Overview

Questions

How can I work with relational datasets?
How can I access remote data directly in my scripts?

Objectives

Learn about JSON format
How to create dictionaries
Loading JSON data and working with it
Using web API’s for accessing remote data

In previous lessons we have learnt about lists and numpy multi-dimensional arrays, which are designed for working with structured, tabular, datasets. But much of the data that we use in our day to day lives, such as that in data catalogs or the communications between modern digital services, does not fit nicely into these tabular datasets. Instead unstructured data formats, which use labels or ‘keys’ to identify each data object are needed. One of the most common formats for such data is the JavaScript Object Notation (JSON) file format. This format was originally developed to fulfil the need for a self-contained, flexible format for real-time server-to-browser communication, and is now used as the basis for many unstructured data formats.

One example of such usage in research is the storage of metadata for data, programs, workflows, or any other such object in a Research Object Crate (RO-Crate). These metadata records take the form:

JSON

{
  "@context": "https://w3id.org/ro/crate/1.1/context",
  "@graph": [
    {
      "@id": "ro-crate-metadata.json",
      "@type": "CreativeWork",
      "about": {
        "@id": "./"
      },
      "conformsTo": {
        "@id": "https://w3id.org/ro/crate/1.1"
      }
    },
    {
      "@id": "./",
      "@type": "Dataset",
      "mainEntity": {
        "@id": "tracking_workflow.ga"
      },
      "hasPart": [
        {
          "@id": "tracking_workflow.ga"
        },
        {
          "@id": "object_tracking_pipeline.png"
        }
      ],
      "author": [

      ],
      "provider": [
        {
          "@id": "#project-1"
        }
      ],
      "license": "Apache-2.0",
      "sdPublisher": {
        "@id": "#person-1"
      },
      "sdDatePublished": "2021-01-01 00:00:00 +0000"
    },
    {
      "@id": "#galaxy",
      "@type": "ComputerLanguage",
      "name": "Galaxy",
      "identifier": {
        "@id": "https://galaxyproject.org/"
      },
      "url": {
        "@id": "https://galaxyproject.org/"
      }
    },
    {
      "@id": "#project-1",
      "@type": "Organization",
      "name": "Science Workflows",
    },
    {
      "@id": "#person-1",
      "@type": "Person",
      "name": "Alice Smith",
    }
  ]
}

The layout of this object is organised using key:value pairs, where the key is a unique string, and the value can be any data type, including other data structures. This simple layout allows quite complex data objects to be constructed.

Dictionary

In python this structure is implemented using the ‘dictionary’ object. Below we will go through the principles of creating and working with these objects. Then we will introduce a library for working with JSON files.

Creation

Lists are created by using square brackets [ ].

Dictionaries are created by using curly brackets { }, e.g.:

PYTHON

d = {}

The simplest way to create a dictionary with some value is:

PYTHON

d = {'keyname': 'keyvalue'}

Following the previous example, we can create a python dictionary using the name of a person as the key and their age as the value:

PYTHON

d = {'alice': 35, 'bob': 18}

PYTHON

print(d)

OUTPUT

{'alice': 35, 'bob': 18}

Alternatively, a dictionary object can be created using the dict function, in a similar manner to using the list function. When using the dict function we need to indicate which key is associated with which value. This can be done in a number of ways, firstly with tuples:

PYTHON

d2 = dict([('alice', 35), ('jane', 24), ('bob',18)])

or with direct association:

PYTHON

d3 = dict(bob=18, alice=35, jane=24)

or using the special zip function, which can be used to create a set of tuples from the given iterable lists:

PYTHON

d4 = dict(zip(['jane','alice','bob'],[24,35,18]))

Accessing elements

To access an element of the dictionary we must use the key:

PYTHON

print('The age of alice is :', d['alice'])

OUTPUT

The age of alice is: 35

We can also use a variable to index the dictionary:

PYTHON

key = 'alice'
print('The name of the person is used as key:', key)
print('The value associated to that key is:', d[key])

OUTPUT

The name of the person is used as key: alice
The value associated to that key is: 35

Adding an element

Adding an element to a dictionary is done by creating a new key and attaching a value to it.

PYTHON

print('Original dictionary:', d)
d['jane'] = 24
print('New dictionary:', d)

OUTPUT

Original dictionary: {'alice': 35, 'bob': 18}
New dictionary: {'alice': 35, 'bob': 18, 'jane': 24}

To add one or more new elements we can also use the update method:

PYTHON

d_extra = {'tom': 54, 'david': 87}

d.update(d_extra)
print('Updated dictionary:', d)

OUTPUT

Updated dictionary: {'alice': 35, 'bob': 18, 'jane': 24, 'tom': 54, 'david': 87}

Dictionary Concatenate Warning

Unlike lists it is not possible to use the + operator to concatenate dictionaries:

PYTHON

{'alice': 35} + {'bob': 18}

OUTPUT

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-39-a6305e6df312> in <module>
----> 1 {'alice': 35} + {'bob': 18}

TypeError: unsupported operand type(s) for +: 'dict' and 'dict'

Key Uniqueness Warning

Keys have to be unique; you cannot have two keys with the same name. If you try to add an item using a key already present in the dictionary you will overwrite the previous value.

PYTHON

print('Original dictionary:', d)
d['alice'] = 12
print('New dictionary:', d)

OUTPUT

Original dictionary: {'alice': 35, 'bob': 18, 'jane': 24}
New dictionary: {'alice': 12, 'bob': 18, 'jane': 24}

Equality between dictionaries

To be equal, all the elements which compose the first dictionary must be present in the second, and only those elements.

The position (ordering) is not important.

PYTHON

d1 = {'alice': 12, 'bob': 18, 'jane': 24, 'tom': 54, 'david': 87}
d2 = {'tom': 54, 'david': 87}
d3 = {'bob': 18, 'alice': 35, 'jane': 24}
d4 = {'alice': 35, 'bob': 18, 'jane': 24}
print('Dictionary 1 and dictionary 2 are equal:', d1 == d2)
print('Dictionary 1 and dictionary 3 are equal:', d1 == d3)
print('Dictionary 3 and dictionary 4 are equal:', d3 == d4)

OUTPUT

Dictionary 1 and dictionary 2 are equal: False
Dictionary 1 and dictionary 3 are equal: False
Dictionary 3 and dictionary 4 are equal: True

Splitting out keys and values

Dictionaries have some special methods. Two of the most useful are keys and values. These return the keys and the values of the dictionary respectively.

PYTHON

d.keys()

OUTPUT

dict_keys(['alice', 'bob', 'jane', 'tom', 'david'])

PYTHON

d.values()

OUTPUT

dict_values([12, 18, 24, 54, 87])

Note that the dict_keys and dict_values objects are iterable but are not lists. This means that they can be used somewhere like a for loop but you can not index them directly.

PYTHON

d.values()[0]

OUTPUT

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'dict_keys' object is not subscriptable

If you want to index keys or values directly, you can convert them to lists with the list function.

PYTHON

list(d.values())[0]

OUTPUT

Presence (or not) of an element inside a dictionary

It is possible to test if a key is present in the dictionary (or not) using the keyword in, just as we did at the start of this lesson for values within a list:

PYTHON

'alice' in d

OUTPUT

True

PYTHON

'mark' in d

OUTPUT

False

Note, however, that we can’t directly test for the presence of values:

PYTHON

12 in d

OUTPUT

False

Instead we would have to use the values method to search these:

PYTHON

12 in d.values()

OUTPUT

True

JSON files

Because JSON files are such a widely used format, python has a built in package for working with JSON, called json: This package provides the method json.load() to read JSON data from a file and and convert it to a python dictionary:

PYTHON

import json

with open('ro-crate-metadata-1.json') as f:
    data = json.load(f)

The closely related method json.loads() (s for “string”) reads a string containing JSON and turns it into a Python dictionary:

PYTHON

json_string = '{"numbers": [1, 2, 3]}'
d = json.loads(json_string)
d['numbers']

OUTPUT

[1, 2, 3]

HTTP requests

Although lot of information is available on the internet for general use, without automated tools for accessing this data it is difficult to make full use of it. Python has a number of libraries for making HTTP requests, to help with this automation, of which the requests library is the most commonly used. This library provides a streamlined application process interface (API) for carrying out these tasks, and has built in JSON support, for easy digesting of the retrieved data.

The basic interaction for making a HTTP request is:

PYTHON

import requests

source_url='https://api.datacite.org/dois/10.48546/workflowhub.workflow.56.1'
requests.get(source_url)

OUTPUT

<Response [200]>

The HTTP request returns a response code - a value of 200 indicates the request was successful. There are a wide range of possible response codes. Those starting as 2XX generally indicate success, whereas those starting with 4XX indicate a failure of some sort (including the most common: 404 Not Found).

The HTTP request we made returned more than just the response code, there will also be the attached content that we requested. In this case our request was made to an API which returns citation information associated with the DOI 10.48546/workflowhub.workflow.56.1. Rather than being presented as a complex webpage, this information is returned as a machine-readable string, similar to the JSON file we read earlier, so we can read this in a similar manner:

PYTHON

response = requests.get(source_url)
record = response.json()

Once the data is in a dictionary we can start exploring it - first step is to check the keys available:

PYTHON

record.keys()

OUTPUT

dict_keys(['data'])

The upper level of the dictionary is simply data - so we can move to the second level:

PYTHON

record['data'].keys()

OUTPUT

dict_keys(['id', 'type', 'attributes', 'relationships'])

The id contains the DOI that we used to find this entry, while the attributes contains the metadata for the object referred to by the DOI. By digging further into the dictionary we can extract information about the object.

Find the title

What is the path to find the title of the object?

solution

PYTHON

record['data']['attributes']['titles'][0]['title']

Find the title (part 2)

Assuming that all JSON objects returned by this API follow the same layout as this record, write a simple function that will return the title of any DOI it is given.

Test that your function works using the DOI: 10.5281/zenodo.4416028

solution

PYTHON

def doi_title( doi_string ):
    source_url = f'https://api.datacite.org/dois/{doi_string}'
    response = requests.get(source_url)
    record = response.json()
    return(record['data']['attributes']['titles'][0]['title'])

Testing the function:

PYTHON

doi_title('10.5281/zenodo.4416028')

OUTPUT

'Britain Breathing 2016-2019 Air Quality and Meteorological Dataset'

Speed Tests…

Sequences are a great tool but they have one big limitation. The execution time to find one specific value inside is linear, as can be shown by fruitless searches for the string x within increasingly long lists of integers.

PYTHON

short_list = list(range(100_000))
long_list = list(range(1_000_000))

We use the built-in %timeit function, to test the speed of these searches:

PYTHON

%timeit -n100 'x' in short_list

OUTPUT

2.02 ms ± 529 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)

PYTHON

%timeit -n100 'x' in long_list

OUTPUT

17.4 ms ± 1.35 ms per loop (mean ± std. dev. of 7 runs, 100 loops each)

Note that the increase in search time is (very roughly) linear.

This is a real problem because the membership test is a very useful and common procedure. So we would like to have something which is not dependent on the number of elements.

Testing access time for large dictionaries

Create two dictionaries, one with 100,000 key:value pairs, the other with 1,000,000 key:value pairs, using the lists created at the start of this lesson. Then use these to test the access times for dictionaries using the %timeit function. How do the access times compare with those for the lists, are they quicker or slower, and do the access times scale linearly with the size of the dictionary?

Show me the solution

The dictionaries can be created using the zip method:

PYTHON

short_hash = dict(zip(short_list,short_list))
long_hash = dict(zip(long_list,long_list))

The access times for the dictionaries are 100-1,000 times faster than for lists, and the search time does not increase with dictionary size.

BASH

%timeit -n100 'x' in short_hash

OUTPUT

34.2 ns ± 2.31 ns per loop (mean ± std. dev. of 7 runs, 100 loops each)

BASH

%timeit -n100 'x' in long_hash

OUTPUT

48.4 ns ± 5.39 ns per loop (mean ± std. dev. of 7 runs, 100 loops each)

Note: it is likely that your first report back from the test will look like this:

OUTPUT

The slowest run took 308.65 times longer than the fastest. This could mean that an intermediate result is being cached.
2.52 µs ± 5.91 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)

This is because the secret behind the speed of searching a dictionary is the caching of the keys after the first access to them. Running the tests a second time will give above test results.

This caching behaviour is very useful for datasets for are accessed regularly.

Key Points

JSON is simple
Dictionaries are defined using key:value pairs
Dictionaries can be nested, and mixed with lists
Web API’s can be accessed using the requests library