Notes to File Away

In this chapter, you learned the following:

• When files are opened and read, their contents are commonly stored in lists of strings.

• Data stored in files is usually formatted in one of a small number of ways, from one value per line to multiline records with explicit end-of-record markers. Each format can be processed stereotypically.

• Data processing programs should be broken into input, processing, and output stages so that each can be reused independently.

• Files can be read (content retrieved), written to (content replaced), and added to (new content appended). When a file is opened in writing mode and it doesn’t exist, a new file is created.

• Data files come in many different formats, so custom code is often required, but you can reuse as much as possible by writing helper functions.

• To make the functions usable by different types of readers, the reader (for a file or URL) is opened outside the function, passed as an argument to the function, and then closed outside the function.

• typing.TextIO is used in type annotations to indicate an open file.

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