[Tut] Dict to List — How to Convert a Dictionary to a List in Python

[Tut] Dict to List — How to Convert a Dictionary to a List in Python - Printable Version

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[Tut] Dict to List — How to Convert a Dictionary to a List in Python - xSicKxBot - 06-30-2020

Dict to List — How to Convert a Dictionary to a List in Python

Summary: To convert a dictionary to a list of tuples, use the

Code:
dict.items()

method to obtain an iterable of

Code:
(key, value)

pairs and convert it to a list using the

Code:
list(...)

constructor:

Code:
list(dict.items())

. To modify each key value pair before storing it in the list, you can use the list comprehension statement

Code:
[(k', v') for k, v in dict.items()]

replacing

Code:
k'

and

Code:
v'

with your specific modifications.

In my code projects, I often find that choosing the right data structure is an important prerequisite to writing clean and effective code. In this article, you’ll learn the most Pythonic way to convert a dictionaryto a list.

Problem: Given a dictionary of

Code:
key:value

pairs. Convert it to a list of

Code:
(key, value)

tuples.

Example: Given the following dictionary.

Code:
d = {'Alice': 19, 'Bob': 23, 'Carl': 47}

You want to convert it to a list of

Code:
(key, value)

tuples:

Code:
[('Alice', 19), ('Bob', 23), ('Carl', 47)]

You can get a quick overview of the methods examined in this article next:

https://trinket.io/embed/python/63517365f5

Exercise: Change the data structure of the dictionary elements. Does it still work?

Let’s dive into the methods!

Method 1: List of Tuples with dict.items() + list()

[Image: dict_to_lists-1024x576.jpg]

The first approach uses the dictionary method

Code:
dict.items()

to retrieve an iterable of

Code:
(key, value)

tuples. The only thing left is to convert it to a list using the built-in

Code:
list()

constructor.

Code:
d = {'Alice': 19, 'Bob': 23, 'Carl': 47} # Method 1
t = list(d.items())
print(t)
# [('Alice', 19), ('Bob', 23), ('Carl', 47)]

The variable

Code:
t

now holds a list of

Code:
(key, value)

tuples. Note that in many cases, it’s not necessary to actually convert it to a list, and, thus, instantiate the data structure in memory. For example, if you want to loop over all

Code:
(key, value)

pairs in the dictionary, you can do so without conversion:

Code:
for k,v in d.items(): s = str(k) + '->' + str(v) print(s) '''
Alice->19
Bob->23
Carl->47 '''

Using the

Code:
items()

method on the dictionary object is the most Pythonic way if everything you want is to retrieve a list of

Code:
(key, value)

pairs. However, what if you want to get a list of keys—ignoring the values for now?

Method 2: List of Keys with dict.key()

To get a list of key values, use the

Code:
dict.keys()

method and pass the resulting iterable into a

Code:
list()

constructor.

Code:
d = {'Alice': 19, 'Bob': 23, 'Carl': 47} # Method 2
t = list(d.keys())
print(t)
# ['Alice', 'Bob', 'Carl']

Similarly, you may want to get a list of values.

Method 3: List of Values with dict.values()

To get a list of key values, use the

Code:
dict.values()

method and pass the resulting iterable into a

Code:
list()

constructor.

Code:
d = {'Alice': 19, 'Bob': 23, 'Carl': 47} # Method 3
t = list(d.values())
print(t)
# [19, 23, 47]

But what if you want to modify each

Code:
(key, value)

tuple? Let’s study some alternatives.

Method 4: List Comprehension with dict.items()

List comprehension is a compact way of creating lists. The simple formula is

Code:
[expression + context]

Expression: What to do with each list element?
Context: What elements to select? The context consists of an arbitrary number of
Code:
for
and
Code:
if
statements.

You can use list comprehension to modify each

Code:
(key, value)

pair from the original dictionary before you store the result in the new list.

Code:
d = {'Alice': 19, 'Bob': 23, 'Carl': 47} # Method 4
t = [(k[:3], v-1) for k, v in d.items()]
print(t)
# [('Ali', 18), ('Bob', 22), ('Car', 46)]

You transform each key to a string with three characters using slicingand reduce each value by one.

Method 5: zip() with dict.keys() and dict.values()

Just for comprehensibility, you could (theoretically) use the

Code:
zip()

functionto create a list of tuples:

Code:
d = {'Alice': 19, 'Bob': 23, 'Carl': 47} # Method 5
t = list(zip(d.keys(), d.values()))
print(t)
# [('Alice', 19), ('Bob', 23), ('Carl', 47)]

However, there’s no benefit compared to just using the

Code:
dict.items()

method. However, I wanted to show you this because the

Code:
zip()

function is frequently used in Python and it’s important for you to understand it.

Method 6: Basic Loop

The last method uses a basic for loop—not the worst way of doing it! Sure, a Python pro would use the most Pythonic ways I’ve shown you above. But using a basic for loop is sometimes superior—especially if you want to be able to customize the code later (e.g., increasing the complexity of the loop body).

Code:
d = {'Alice': 19, 'Bob': 23, 'Carl': 47} # Method 6
t = []
for k, v in d.items(): t.append((k,v))
print(t)
# [('Alice', 19), ('Bob', 23), ('Carl', 47)]

A single-line for loop or list comprehension statement is not the most Pythonic way to convert a dictionary to a Python list if you want to modify each new list element using a more complicated body expression. In this case, a straightforward for loop is your best choice!

Related articles:

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