add: str.partition post

_posts/2024-01-22-partition-method-of-python-str.md

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+---
+title: Partition method of Python strings
+date: 2024-01-22 16:55:00 +05:30
+categories: [Python, Snippets, str]
+tags: [python, str]     # TAG names should always be lowercase
+---
+
+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.
+
+```python
+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.
+
+```python
+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](https://github.com/pallets/flask/blob/7b5e176d1ab23e183d0403573358299ed6562dce/src/flask/app.py#L588). 
+
+A simplified version of the code is added as below:
+
+```python
+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.
+
+---
+
+I share interesting Python snippets 🐍 like this from open-source projects illustrating Python language features. Subscribe to [my newsletter](https://adarshd.substack.com/) to receive new posts to your email 💌🚀.