Python 3.10: Cool New Features for You to Try

Deconstructing Data Structures

At its core, structural pattern matching is about defining patterns to which your data structures can be matched. In this section, you’ll study a practical example where you’ll work with data that are structured differently, even though the meaning is the same. You’ll define several patterns, and depending on which pattern matches your data, you’ll process your data appropriately.

This section will be a bit light on explanations of the possible patterns. Instead, it will try to give you an impression of the possibilities. The next section will step back and explain the patterns in more detail.

Time to match your first pattern! The following example uses a match ... case block to find the first name of a user by extracting it from a user data structure:

>>> user = {
...     "name": {"first": "Pablo", "last": "Galindo Salgado"},
...     "title": "Python 3.10 release manager",
... }

>>> match user:
...     case {"name": {"first": first_name}}:
...         pass
...

>>> first_name
'Pablo'

You can see structural pattern matching at work in the highlighted lines. user is a small dictionary with user information. The case line specifies a pattern that user is matched against. In this case, you’re looking for a dictionary with a "name" key whose value is a new dictionary. This nested dictionary has a key called "first". The corresponding value is bound to the variable first_name.

For a practical example, say that you’re processing user data where the underlying data model changes over time. Therefore, you need to be able to process different versions of the same data.

In the next example, you’ll use data from randomuser.me. This is a great API for generating random user data that you can use during testing and development. The API is also an example of an API that has changed over time. You can still access the old versions of the API.

You may expand the collapsed section below to see how you can use requests to obtain different versions of the user data using the API:

# random_user.py

import requests

def get_user(version="1.3"):
    """Get random users"""
    url = f"https://randomuser.me/api/{version}/?results=1"
    response = requests.get(url)
    if response:
        return response.json()["results"][0]

{
    "gender": "female",
    "name": {
        "title": "Miss",
        "first": "Ilona",
        "last": "Jokela"
    },
    "location": {
        "street": {
            "number": 4473,
            "name": "Mannerheimintie"
        },
        "city": "Harjavalta",
        "state": "Ostrobothnia",
        "country": "Finland",
        "postcode": 44879,
        "coordinates": {
            "latitude": "-6.0321",
            "longitude": "123.2213"
        },
        "timezone": {
            "offset": "+5:30",
            "description": "Bombay, Calcutta, Madras, New Delhi"
        }
    },
    "email": "ilona.jokela@example.com",
    "login": {
        "uuid": "632b7617-6312-4edf-9c24-d6334a6af52d",
        "username": "brownsnake482",
        "password": "biatch",
        "salt": "ofk518ZW",
        "md5": "6d589615ca44f6e583c85d45bf431c54",
        "sha1": "cd87c931d579bdff77af96c09e0eea82d1edfc19",
        "sha256": "6038ede83d4ce74116faa67fb3b1b2e6f6898e5749b57b5a0312bd46a539214a"
    },
    "dob": {
        "date": "1957-05-20T08:36:09.083Z",
        "age": 64
    },
    "registered": {
        "date": "2006-07-30T18:39:20.050Z",
        "age": 15
    },
    "phone": "07-369-318",
    "cell": "048-284-01-59",
    "id": {
        "name": "HETU",
        "value": "NaNNA204undefined"
    },
    "picture": {
        "large": "https://randomuser.me/api/portraits/women/28.jpg",
        "medium": "https://randomuser.me/api/portraits/med/women/28.jpg",
        "thumbnail": "https://randomuser.me/api/portraits/thumb/women/28.jpg"
    },
    "nat": "FI"
}

url = f"https://randomuser.me/api/{version}/?results=1&seed=310"

{
    "gender": "female",
    "name": {
        "title": "miss",
        "first": "ilona",
        "last": "jokela"
    },
    "location": {
        "street": "7336 myllypuronkatu",
        "city": "kurikka",
        "state": "central ostrobothnia",
        "postcode": 53740
    },
    "email": "ilona.jokela@example.com",
    "login": {
        "username": "blackelephant837",
        "password": "sand",
        "salt": "yofk518Z",
        "md5": "b26367ea967600d679ee3e0b9bda012f",
        "sha1": "87d2910595acba5b8e8aa8b00a841bab08580e2f",
        "sha256": "73bd0d205d0dc83ae184ae222ff2e9de5ea4039119a962c4f97fabd5bbfa7aca"
    },
    "dob": "1966-04-17 11:57:01",
    "registered": "2005-08-10 10:15:01",
    "phone": "04-636-931",
    "cell": "048-828-40-15",
    "id": {
        "name": "HETU",
        "value": "366-9204"
    },
    "picture": {
        "large": "https://randomuser.me/api/portraits/women/24.jpg",
        "medium": "https://randomuser.me/api/portraits/med/women/24.jpg",
        "thumbnail": "https://randomuser.me/api/portraits/thumb/women/24.jpg"
    },
    "nat": "FI"
}

In this example, you’ll work with the information about the date of birth (dob) for each user. The structure of these data has changed between different versions of the Random User API:

# Version 1.1
"dob": "1966-04-17 11:57:01"

# Version 1.3
"dob": {"date": "1957-05-20T08:36:09.083Z", "age": 64}

Note that in version 1.1, the date of birth is represented as a simple string, while in version 1.3, it’s a JSON object with two members: "date" and "age". Say that you want to find the age of a user. Depending on the structure of your data, you’d either need to calculate the age based on the date of birth or look up the age if it’s already available.

Traditionally, you would detect the structure of the data with an if test, maybe based on the type of the "dob" field. You can approach this differently in Python 3.10. Now, you can use structural pattern matching instead:

 1# random_user.py (continued)
 2
 3from datetime import datetime
 4
 5def get_age(user):
 6    """Get the age of a user"""
 7    match user:
 8        case {"dob": {"age": int(age)}}:
 9            return age
10        case {"dob": dob}:
11            now = datetime.now()
12            dob_date = datetime.strptime(dob, "%Y-%m-%d %H:%M:%S")
13            return now.year - dob_date.year

The match ... case construct is new in Python 3.10 and is how you perform structural pattern matching. You start with a match statement that specifies what you want to match. In this example, that’s the user data structure.

One or several case statements follow match. Each case describes one pattern, and the indented block beneath it says what should happen if there’s a match. In this example:

• Line 8 matches a dictionary with a "dob" key whose value is another dictionary with an integer (int) item named "age". The name age captures its value.

• Line 10 matches any dictionary with a "dob" key. The name dob captures its value.

One important feature of pattern matching is that at most one pattern will be matched. Since the pattern on line 10 matches any dictionary with "dob", it’s important that the more specific pattern on line 8 comes first.

Before looking closer at the details of the patterns and how they work, try calling get_age() with different data structures to see the result:

>>> import random_user

>>> users11 = random_user.get_user(version="1.1")
>>> random_user.get_age(users11)
55

>>> users13 = random_user.get_user(version="1.3")
>>> random_user.get_age(users13)
64

Your code can calculate the age correctly for both versions of the user data, which have different dates of birth.

Look closer at those patterns. The first pattern, {"dob": {"age": int(age)}}, matches version 1.3 of the user data:

{
    ...
    "dob": {"date": "1957-05-20T08:36:09.083Z", "age": 64},
    ...
}

The first pattern is a nested pattern. The outer curly braces say that a dictionary with the key "dob" is required. The corresponding value should be a dictionary. This nested dictionary must match the subpattern {"age": int(age)}. In other words, it needs to have an "age" key with an integer value. That value is bound to the name age.

The second pattern, {"dob": dob}, matches the older version 1.1 of the user data:

{
    ...
    "dob": "1966-04-17 11:57:01",
    ...
}

This second pattern is a simpler pattern than the first one. Again, the curly braces indicate that it will match a dictionary. However, any dictionary with a "dob" key is matched because there are no other restrictions specified. The value of that key is bound to the name dob.

The main takeaway is that you can describe the structure of your data using mostly familiar notation. One striking change, though, is that you can use names like dob and age, which aren’t yet defined. Instead, values from your data are bound to these names when a pattern matches.

You’ve explored some of the power of structural pattern matching in this example. In the next section, you’ll dive a bit more into the details.

Using Different Kinds of Patterns

You’ve seen an example of how you can use patterns to effectively unravel complicated data structures. Now, you’ll take a step back and look at the building blocks that make up this new feature. Many things come together to make it work. In fact, there are three Python Enhancement Proposals (PEPs) that describe structural pattern matching:

1. PEP 634: Specification
2. PEP 635: Motivation and Rationale
3. PEP 636: Tutorial

These documents give you a lot of background and detail if you’re interested in a deeper dive than what follows.

Patterns are at the center of structural pattern matching. In this section, you’ll learn about some of the different kinds of patterns that exist:

• Mapping patterns match mapping structures like dictionaries.
• Sequence patterns match sequence structures like tuples and lists.
• Capture patterns bind values to names.
• AS patterns bind the value of subpatterns to names.
• OR patterns match one of several different subpatterns.
• Wildcard patterns match anything.
• Class patterns match class structures.
• Value patterns match values stored in attributes.
• Literal patterns match literal values.

You already used several of them in the example in the previous section. In particular, you used mapping patterns to unravel data stored in dictionaries. In this section, you’ll learn more about how some of these work. All the details are available in the PEPs mentioned above.

A capture pattern is used to capture a match to a pattern and bind it to a name. Consider the following recursive function that sums a list of numbers:

 1def sum_list(numbers):
 2    match numbers:
 3        case []:
 4            return 0
 5        case [first, *rest]:
 6            return first + sum_list(rest)

The first case on line 3 matches the empty list and returns 0 as its sum. The second case on line 5 uses a sequence pattern with two capture patterns to match lists with one or more elements. The first element in the list is captured and bound to the name first. The second capture pattern, *rest, uses unpacking syntax to match any number of elements. rest will bind to a list containing all elements of numbers except the first one.

sum_list() calculates the sum of a list of numbers by recursively adding the first number in the list and the sum of the rest of the numbers. You can use it as follows:

>>> sum_list([4, 5, 9, 4])
22

The sum of 4 + 5 + 9 + 4 is correctly calculated to be 22. As an exercise for yourself, you can try to trace the recursive calls to sum_list() to make sure you understand how the code sums the whole list.

sum_list() handles summing up a list of numbers. Observe what happens if you try to sum anything that isn’t a list:

>>> print(sum_list("4594"))
None

>>> print(sum_list(4594))
None

Passing a string or a number to sum_list() returns None. This occurs because none of the patterns match, and the execution continues after the match block. That happens to be the end of the function, so sum_list() implicitly returns None.

Often, though, you want to be alerted about failed matches. You can add a catchall pattern as the final case that handles this by raising an error, for example. You can use the underscore (_) as a wildcard pattern that matches anything without binding it to a name. You can add some error handling to sum_list() as follows:

def sum_list(numbers):
    match numbers:
        case []:
            return 0
        case [first, *rest]:
            return first + sum_list(rest)
        case _:
            wrong_type = numbers.__class__.__name__
            raise ValueError(f"Can only sum lists, not {wrong_type!r}")

The final case will match anything that doesn’t match the first two patterns. This will raise a descriptive error, for instance, if you try to calculate sum_list(4594). This is useful when you need to alert your users that some input was not matched as expected.

Your patterns are still not foolproof, though. Consider what happens if you try to sum a list of strings:

>>> sum_list(["45", "94"])
TypeError: can only concatenate str (not "int") to str

The base case returns 0, so therefore the summing only works for types that you can add with numbers. Python doesn’t know how to add numbers and text strings together. You can restrict your pattern to only match integers using a class pattern:

def sum_list(numbers):
    match numbers:
        case []:
            return 0
        case [int(first), *rest]:
            return first + sum_list(rest)
        case _:
            raise ValueError(f"Can only sum lists of numbers")

Adding int() around first makes sure that the pattern only matches if the value is an integer. This might be too restrictive, though. Your function should be able to sum both integers and floating-point numbers, so how can you allow this in your pattern?

To check whether at least one out of several subpatterns match, you can use an OR pattern. OR patterns consist of two or more subpatterns, and the pattern matches if at least one of the subpatterns does. You can use this to match when the first element is either of type int or type float:

def sum_list(numbers):
    match numbers:
        case []:
            return 0
        case [int(first) | float(first), *rest]:
            return first + sum_list(rest)
        case _:
            raise ValueError(f"Can only sum lists of numbers")

You use the pipe symbol (|) to separate the subpatterns in an OR pattern. Your function now allows summing a list of floating-point numbers:

>>> sum_list([45.94, 46.17, 46.72])
138.82999999999998

There’s a lot of power and flexibility within structural pattern matching, even more than what you’ve seen so far. Some things that aren’t covered in this overview are:

• Using guards to restrict patterns
• Using AS patterns to capture the value of subpatterns
• Using class patterns to match custom enums and data classes

If you’re interested, have a look in the documentation to learn more about these features as well. In the next section, you’ll learn about literal patterns and value patterns.

Matching Literal Patterns

A literal pattern is a pattern that matches a literal object like an explicit string or number. In a sense, this is the most basic kind of pattern and allows you to emulate switch ... case statements seen in other languages. The following example matches a specific name:

def greet(name):
    match name:
        case "Guido":
            print("Hi, Guido!")
        case _:
            print("Howdy, stranger!")

The first case matches the literal string "Guido". In this case, you use _ as a wildcard to print a generic greeting whenever name is not "Guido". Such literal patterns can sometimes take the place of if ... elif ... else constructs and can play the same role that switch ... case does in some other languages.

One limitation with structural pattern matching is that you can’t directly match values stored in variables. Say that you’ve defined bdfl = "Guido". A pattern like case bdfl: will not match "Guido". Instead, this will be interpreted as a capture pattern that matches anything and binds that value to bdfl, effectively overwriting the old value.

You can, however, use a value pattern to match stored values. A value pattern looks a bit like a capture pattern but uses a previously defined dotted name that holds the value that will be matched against.

You can, for example, use an enumeration to create such dotted names:

import enum

class Pythonista(str, enum.Enum):
    BDFL = "Guido"
    FLUFL = "Barry"

def greet(name):
    match name:
        case Pythonista.BDFL:
            print("Hi, Guido!")
        case _:
            print("Howdy, stranger!")

The first case now uses a value pattern to match Pythonista.BDFL, which is "Guido". Note that you can use any dotted name in a value pattern. You could, for example, have used a regular class or a module instead of the enumeration.

To see a bigger example of how to use literal patterns, consider the game of FizzBuzz. This is a counting game where you should replace some numbers with words according to the following rules:

• You replace numbers divisible by 3 with fizz.
• You replace numbers divisible by 5 with buzz.
• You replace numbers divisible by both 3 and 5 with fizzbuzz.

FizzBuzz is sometimes used to introduce conditionals in programming education and as a screening problem in interviews. Even though a solution is quite straightforward, Joel Grus has written a full book about different ways to program the game.

A typical solution in Python will use if ... elif ... else as follows:

def fizzbuzz(number):
    mod_3 = number % 3
    mod_5 = number % 5

    if mod_3 == 0 and mod_5 == 0:
        return "fizzbuzz"
    elif mod_3 == 0:
        return "fizz"
    elif mod_5 == 0:
        return "buzz"
    else:
        return str(number)

The % operator calculates the modulus, which you can use to test divisibility. Namely, if a modulus b is 0 for two numbers a and b, then a is divisible by b.

In fizzbuzz(), you calculate number % 3 and number % 5, which you then use to test for divisibility with 3 and 5. Note that you must do the test for divisibility with both 3 and 5 first. If not, numbers that are divisible by both 3 and 5 will be covered by either the "fizz" or the "buzz" cases instead.

You can check that your implementation gives the expected result:

>>> fizzbuzz(3)
fizz

>>> fizzbuzz(14)
14

>>> fizzbuzz(15)
fizzbuzz

>>> fizzbuzz(92)
92

>>> fizzbuzz(65)
buzz

You can confirm for yourself that 3 is divisible by 3, 65 is divisible by 5, and 15 is divisible by both 3 and 5, while 14 and 92 aren’t divisible by either 3 or 5.

An if ... elif ... else structure where you’re comparing one or a few variables several times over is quite straightforward to rewrite using pattern matching instead. For example, you can do the following:

def fizzbuzz(number):
    mod_3 = number % 3
    mod_5 = number % 5

    match (mod_3, mod_5):
        case (0, 0):
            return "fizzbuzz"
        case (0, _):
            return "fizz"
        case (_, 0):
            return "buzz"
        case _:
            return str(number)

You match on both mod_3 and mod_5. Each case pattern then matches either the literal number 0 or the wildcard _ on the corresponding values.

Compare and contrast this version with the previous one. Note how the pattern (0, 0) corresponds to the test mod_3 == 0 and mod_5 == 0, while (0, _) corresponds to mod_3 == 0.

As you saw earlier, you can use an OR pattern to match on several different patterns. For example, since mod_3 can only take the values 0, 1, and 2, you can replace case (_, 0) with case (1, 0) | (2, 0). Remember that (0, 0) has already been covered.

The Python core developers have consciously chosen not to include switch ... case statements in the language earlier. However, there are some third-party packages that do, like switchlang, which