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.

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. It returns a three-membered tuple containing the part before, the separator itself (if a match is found, else empty string), and the part after the 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 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

Here, the `server_name` string is partitioned into host and port values.

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