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Using Text and Binary Files

This section expands on the impact of Python 3.0’s string model on the file processing basics introduced earlier in the book. As mentioned earlier, the mode in which you open a file is crucial—it determines which object type you will use to represent the file’s content in your script. Text mode implies str objects, and binary mode implies bytes objects:

 

Text-mode files interpret file contents according to a Unicode encoding—either the default for your platform, or one whose name you pass in. By passing in an encoding name to open, you can force conversions for various types of Unicode files. Text-mode files also perform universal line-end translations: by default, all line-end forms map to the single '\n' character in your script, regardless of the platform on which you run it. As described earlier, text files also handle reading and writing the byte order mark (BOM) stored at the start-of-file in some Unicode encoding schemes.Binary-mode files instead return file content to you raw, as a sequence of integers representing byte values, with no encoding or decoding and no line-end translations.

The second argument to open determines whether you want text or binary processing, just as it does in 2.X Python—adding a “b” to this string implies binary mode (e.g., "rb" to read binary data files). The default mode is "rt"; this is the same as "r", which means text input (just as in 2.X).

In 3.0, though, this mode argument to open also implies an object type for file content representation, regardless of the underlying platform—text files return a str for reads and expect one for writes, but binary files return a bytes for reads and expect one (or a bytearray) for writes.