85 lines
2.6 KiB
Markdown
85 lines
2.6 KiB
Markdown
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---
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tags: [python]
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created: Sunday, March 16, 2025
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---
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# Using a context manager in Python
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For certain process that have a start and an end state such as opening and
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closing a file or connecting and disconnecting from a database, instead of
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having dedicated open/close, connect/disconnect handling, you can use a Context
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Manager as a form of "syntactic sugar".
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By using a Context Manager you can use a more Pythonic construction of e.g:
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```py
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with open('/file-path') as file:
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... # handle file operation
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```
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```py
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with open DatabaseService() as connection:
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... # do database stuff
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```
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When you use this approach it is an abstraction over `try...finally`. Meaning
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the clean-up `finally` operation will run automatically without you having to
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explicitly handle it.
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Some common processes such as file handling have the Context Manager built-in
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and you don't have to explicitly provision it.
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Other processes may lend themselves to `with` syntax but you have to do the
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configuration yourself.
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One such example is creating a database accessor. Below I have done this by
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using `__enter__` and `__exit__` methods on a database class:
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```py
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class DatabaseService:
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def __init__(self, db_name, db_path):
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self.db_name = db_name
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self.db_path = db_path
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self.connection: Optional[sqlite3.Connection] = None
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def connect(self) -> Optional[sqlite3.Connection]:
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if self.connection is not None:
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return self.connection
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try:
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if not os.path.exists(self.db_path):
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os.makedirs(self.db_path)
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print("INFO Created database directory")
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self.connection = sqlite3.connect(f"{self.db_path}/{self.db_name}.db")
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self.connection.execute("PRAGMA foreign_keys = ON")
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return self.connection
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except Exception as e:
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raise Exception(f"ERROR Problem connecting to database: {e}")
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def disconnect(self) -> None:
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try:
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if self.connection is not None:
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self.connection.close()
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self.connection = None
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except Exception as e:
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raise Exception(f"ERROR Problem disconnecting from database: {e}")
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def __enter__(self) -> sqlite3.Connection:
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connection = self.connect()
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if connection is None:
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raise RuntimeError("Failed to establish database connection")
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return connection
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def __exit__(self) -> None:
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self.disconnect()
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```
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Then I can use it like so:
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```py
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with DatabaseService() as connection:
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cursor = connection.cursor()
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cursor.execute(...)
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```
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