What are the Differences Between Python's File Opening Modes (r, r , w, w , a, a )?

Understanding File Opening Modes in Python's Open Function

The built-in Python open() function offers various file opening modes, allowing users to specify the desired access and usage for a file. One may encounter several modes such as a, a , w, w , and r , but their exact functionalities and differences may not be immediately apparent.

Mode Comparison: Writing and Updating

These modes are primarily used for writing or updating files. The mode w truncates any existing file to zero length or creates a new one for writing. The file stream is positioned at the beginning, allowing overwriting of any existing content. The w mode similarly allows both reading and writing, but if the file does not exist, it is created and truncated.

Mode Comparison: Appending

The a mode opens a file for writing, creating it if it does not exist. The file stream is positioned at the end of the file. Subsequent writes using this mode will always append to the current end of the file, regardless of any intervening file manipulations such as fseek(). The a mode adds the ability to read from the file while maintaining the append-only behavior.

In-Depth Definition

To fully understand the specific semantics of these modes, it is helpful to refer to the documentation of the C standard library function fopen(), as Python's open() function follows the same mode definitions. According to the BSD fopen manpage, the modes are defined as follows:

• r: Open for reading from the beginning of the file.
• r : Open for reading and writing from the beginning.
• w: Truncate or create for writing from the beginning.
• w : Open for reading and writing after truncation or creation.
• a: Open for writing at the end of the file.
• a : Open for reading and writing at the end of the file.

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