Dictionaries

Last updated on 2025-11-10 | Edit this page

Estimated time: 20 minutes

Overview

Questions

  • How can I store multiple values that are not indexed by position?

Objectives

Based on Toby Hodges’ dictionary chapter in the EMBL ITPP course.

Looking up data

Lists store many values together; this is fine if you are really really careful and you don’t need to change the arrays that much. Otherwise, it is prone to errors. Dictionaries are similar to lists, but instead of holding an ordered collection of values, they hold key-value pairs.

  • Use a dictionary to store many key-value pairs together.
    • Contained within angled brackets {...}.
    • Values separated by commas ,.
    • key-value pairs connected with colons :.
  • Use len to find out how many key-value pairs are in a dictionary.

PYTHON 

student_numbers = { 'Bioscience Technology': 16, 
                    'Computational Biology': 12,
                    'Post-Genomic Biology': 20,
                    'Ecology and Environmental Management': 3,
                    'Maths in the Living Environment': 0
                  }
print('#students:', student_numbers)
print('length:', len(student_numbers))

OUTPUT 

#students: {'Bioscience Technology': 16, 'Computational Biology': 12, 'Post-Genomic Biology': 20, 'Ecology and Environmental Management': 3, 'Maths in the Living Environment': 0}
length: 5

Use a value’s key to fetch it from a dictionary

PYTHON 

print('#students in Bioscience Technology:', student_numbers['Bioscience Technology'])
print('#students in Post-Genomic Biology:', student_numbers['Post-Genomic Biology'])

OUTPUT 

#students in Bioscience Technology: 16
#students in Post-Genomic Biology: 20

There is also a method called get() that will give you the same result:

PYTHON 

print('#students in Bioscience Technology:', student_numbers.get('Bioscience Technology'))

OUTPUT 

#students in Bioscience Technology: 16

Get Keys

The keys() method will return a list of all the keys in the dictionary.

PYTHON 

student_numbers.keys()

OUTPUT 

dict_keys(['Bioscience Technology', 'Computational Biology', 'Post-Genomic Biology', 'Ecology and Environmental Management', 'Maths in the Living Environment'])

Get Values

Similarly, the values() method will return a list of all the values in the dictionary.

PYTHON 

student_numbers.values()

OUTPUT 

dict_values([16, 12, 20, 3, 0])
Callout

Both? Why not both?

If you wish to get keys and values at the same time (for example to iterate over them and do something with each key/value pair), you can use the items() method. We will revisit this later, when we talk about loops.

Callout

Dictionaries are ordered

The way that dictionaries are implemented in Python fundamentally changed in v3.6, resulting in them taking up ~1/2 the space and working ~2x as fast as they used to. A side effect of this is that dictionary objects in Python 3.6 remember the order that entries were created in and you should be able to access their entries in this order. Regardless, in the examples and exercises in this course, we assume that this order cannot be relied upon - this is not yet considered a ‘stable’ feature of the language i.e. future versions of Python are not guaranteed to preserve the order of dictionaries. When writing your own code, if you want to access dictionary entries in a particular order, you should make sure to do so by providing keys in a specific order.

Dictionaries’ values can be replaced by assigning to them

  • Use a key on the left of assignment to replace a value.

PYTHON 

student_numbers['Bioscience Technology'] = 2
print('student_numbers is now:', student_numbers)

OUTPUT 

student_numbers is now: {'Bioscience Technology': 2, 'Computational Biology': 12, 'Post-Genomic Biology': 20, 'Ecology and Environmental Management': 3, 'Maths in the Living Environment': 0}

The same effect can be achieved with the update() function:

PYTHON 

student_numbers.update({'Maths in the Living Environment': 120})
print('student_numbers is now:', student_numbers)

OUTPUT 

student_numbers is now: {'Bioscience Technology': 2, 'Computational Biology': 12, 'Post-Genomic Biology': 20, 'Ecology and Environmental Management': 3, 'Maths in the Living Environment': 120}
  • update is a method of dictionaries.
    • Like a function, but tied to a particular object.
  • Use object_name.method_name to call methods.
    • Deliberately resembles the way we refer to things in a library.

Adding items to a dictionary

If you try to assign a value to a key that doesn’t exist, Python creates the entry for you automatically. This is how new items are usually added to the dictionary, by using a new index key and assigning a value to it.

PYTHON 

temperatures = {
    'Monday': 22,
    'Tuesday': 24,
    'Wednesday': 21,
    'Thursday': 23,
    'Friday': 25
}
print('temperatures is initially:', temperatures)

temperatures['Saturday'] = 24
print('temperatures has become:', temperatures)

OUTPUT 

temperatures is initially: {'Monday': 22, 'Tuesday': 24, 'Wednesday': 21, 'Thursday': 23, 'Friday': 25}
temperatures has become: {'Monday': 22, 'Tuesday': 24, 'Wednesday': 21, 'Thursday': 23, 'Friday': 25, 'Saturday': 24}

The same can be achieved with the update() function:

PYTHON 

temperatures.update({'Sunday': 26})
print('temperatures has become: ', temperatures)

OUTPUT 

temperatures has become: {'Monday': 22, 'Tuesday': 24, 'Wednesday': 21, 'Thursday': 23, 'Friday': 25, 'Saturday': 24, 'Sunday': 26}

Like lists, dictionaries can contain values of any type; therefore, dictionaries of lists or dictionaries of dictionaries are possible.

Challenge

Nested dictionaries

Fill in the blanks so that the program below produces the indicated output:

PYTHON 

grades = {'Juan': {'Maths': 2},
          'John': {'History': 1, 'Biology': 3},
          'Gianni': {'Biology': 1}}

grades['Juan'][______] = 2
grades['Juan'][______] = 1

grades[______]['Maths'] = 1

grades[______]['History'] = 4
grades[______][______] = 3

print(grades)

OUTPUT 

{'Juan': {'Maths': 2, 'History': 2, 'Biology': 1}, 'John': {'History': 1, 'Biology': 3, 'Maths': 1}, 'Gianni': {'Biology': 1, 'History': 4, 'Maths': 3}}

You can choose how you want to fill in the gaps; but if we assign the subjects by the order they were introduced, this would be the solution:

PYTHON 

grades = {'Juan': {'Maths': 2},
          'John': {'History': 1, 'Biology': 3},
          'Gianni': {'Biology': 1}}

grades['Juan']['History'] = 2
grades['Juan']['Biology'] = 1

grades['John']['Maths'] = 1

grades['Gianni']['History'] = 4
grades['Gianni']['Maths'] = 3

print(grades)

OUTPUT 

{'Juan': {'Maths': 2, 'History': 2, 'Biology': 1}, 'John': {'History': 1, 'Biology': 3, 'Maths': 1}, 'Gianni': {'Biology': 1, 'History': 4, 'Maths': 3}}

Notice that the subject order reflects the order in which the key/value pairs were added!

Use del to remove items from a dictionary entirely

  • We use del dict_name[key] to remove an element from a dictionary (in the example, ‘Everyday’ is not a day of the week).
  • del is not a function or a method, but a statement in the language.

PYTHON 

temperatures = {
    'Monday': 22,
    'Tuesday': 24,
    'Wednesday': 21,
    'Thursday': 23,
    'Friday': 25,
    'Everyday': 49    
}
print('temperature before removing Everyday:', temperatures)
del temperatures['Everyday']
print('temperature after removing Everyday:', temperatures)

OUTPUT 

temperature before removing Everyday: {'Monday': 22, 'Tuesday': 24, 'Wednesday': 21, 'Thursday': 23, 'Friday': 25, 'Everyday': 49}
temperature after removing Everyday: {'Monday': 22, 'Tuesday': 24, 'Wednesday': 21, 'Thursday': 23, 'Friday': 25}

The empty dictionary contains no values

  • Use {} on its own to represent a dictionary that doesn’t contain any values.
    • “The zero of dictionaries.”
  • Helpful as a starting point for collecting values.

Indexing will not work on dictionaries

  • If we want to retrieve the 4th item of a dictionary, we can’t simply ask for it by index:

PYTHON 

print(temperatures[3])

OUTPUT 

Traceback (most recent call last):
  File "<python-input-19>", line 1, in <module>
    print(temperatures[3])
          ~~~~~~~~~~~~^^^
KeyError: 3

Python reports a KeyError; this indicates that it searched the dictionary’s keys for the key 3 (careful: 3 the integer, not the string “3”), and didn’t find it. If we really, really want the fourth item of the dictionary (see “Dictionaries are ordered” above), we can ask for the value associated with the fourth key:

PYTHON 

keys = list(temperatures.keys())
fourth_key = keys[3]
print('temperature on the fourth day:', temperatures[fourth_key])

OUTPUT 

temperature on the fourth day: 23
Discussion

Practice

Complete the W3Schools training exercises for

Challenge

Nested dictionaries

Consider this syntax:

PYTHON 

a = {'name' : 'John', 'age' : '20'}
b = {'name' : 'May', 'age' : '23'}
customers = {'c1' : a, 'c2' : b}

what will be a correct syntax for printing the name ‘May’?

  • print(customers['c2']['name'])
  • print(customers.c2.b['name'])
  • print(customers.c2.name)

To access a value in a dictionary we need to use its key. May is customer “c2”, so first we need to specify that:

PYTHON 

may_customer = customers['c2']

The variable may_customer contains the value saved under the key 'c2', namely the dictionary b. Within that, the value “May” is saved under the key 'name':

PYTHON 

print(may_customer['name'])

OUTPUT 

May

Let’s put it together:

PYTHON 

print(customers['c2']['name'])

OUTPUT 

May
Key Points
  • A dictionary stores many values as key-value pairs.
  • Use the key of a value to fetch it from a dictionary.
  • Dictionaries’ values can be replaced by assigning to them.
  • Setting a value to a key will update it, if it exists, or create it, if it doesn’t.
  • Use del to remove items from a dictionary entirely.
  • The empty dictionary contains no key/value pairs.
  • Dictionaries may contain keys and values of different types.
  • Trying to use indices to summon key/value pairs is an error.