Partition method of Python strings

The partition method of str covers a niche use case - It is useful when a string needs to be split into exactly two parts based on a separator. It returns a three-membered tuple containing the part before, the separator itself, and the part after the separator.

The split method is commonly used for splitting strings, which returns a list whose length can change based on the number of times the separator is present.

host = "localhost:8000"
print(host.split(":"))  # Output: ['localhost', '8000']

host = "localhost"
print(host.split(":"))  # Output:['localhost']

host = ""
print(host.split(":"))  # Output: ['']

partition is helpful when we need to get precisely two parts from a string using a separator.

server_name = "localhost:5000"
print(server_name.partition(":"))  # Output: ('localhost', ':', '5000')

server_name = "localhost"
print(server_name.partition(":"))  # Output: ('localhost', '', '')

server_name = ""
print(server_name.partition(":"))  # Output: ('', '', '')

Using partition instead of split saves us from additional checks on the result's length when exactly two parts are required.

Flask uses the partition method to determine the host and port to use when running the Flask app. You can see the source code here.

A simplified version of the code is added as below:

def run(
    self,
    host: str | None = None,
    port: int | None = None,
    debug: bool | None = None,
    load_dotenv: bool = True,
    **options: t.Any,
) -> None:
    ...

    server_name = self.config.get("SERVER_NAME")
    sn_host = sn_port = None

    if server_name:
        sn_host, _, sn_port = server_name.partition(":")
        # The `partition` method will always return a three-membered tuple

    if not host:
        if sn_host:
            host = sn_host
        else:
            host = "127.0.0.1"

    if port or port == 0:
        port = int(port)
    elif sn_port:
        port = int(sn_port)
    else:
        port = 5000

In the example above, the server_name string is partitioned into host and port values.

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