Reading/Writing Files & Context Managers

Every program you have written so far has lived and died in memory. Close the program, and everything is gone. Reading and writing files is how a program persists data across runs. This chapter also introduces context managers, a pattern you will recognize again the moment you meet database connections or network sockets later.

`open(path, mode)` returns a file object. The mode controls what you can do with it: `"r"` for reading (the default), `"w"` for writing (creates the file if missing, and overwrites it if it already exists), and `"a"` for appending to the end of an existing file.

Manually calling `.close()` has a real problem: if an exception is raised between `open()` and `.close()`, the `.close()` call never runs, and the file is left open, which can mean data never gets flushed to disk, or the file stays locked for other programs. A `with` block is Python's context manager syntax. It guarantees `.close()` runs automatically when the block exits, whether it exits normally or via an exception.

A file object is itself iterable line by line, which is memory-efficient for large files. You never have to load the whole file into memory at once the way `.read()` does.

• -`"r"` reads (the file must already exist). `"w"` writes, overwriting any existing content. `"a"` appends to the end.
• -`with open(...) as f:` guarantees the file is closed even if an exception occurs inside the block. Always prefer it.
• -`f.read()` loads the whole file as one string. Iterating over `f` (or calling `f.readlines()`) gives it to you line by line.