io — Core tools for working with streams — Python v3.0.1 documentation (original) (raw)
Open file and return a stream. If the file cannot be opened, anIOError is raised.
file is either a string giving the name (and the path if the file isn’t in the current working directory) of the file to be opened or a file descriptor of the file to be opened. (If a file descriptor is given, for example, from os.fdopen(), it is closed when the returned I/O object is closed, unless closefd is set to False.)
mode is an optional string that specifies the mode in which the file is opened. It defaults to 'r' which means open for reading in text mode. Other common values are 'w' for writing (truncating the file if it already exists), and 'a' for appending (which on some Unix systems, means that all writes append to the end of the file regardless of the current seek position). In text mode, if encoding is not specified the encoding used is platform dependent. (For reading and writing raw bytes use binary mode and leave encoding unspecified.) The available modes are:
| Character | Meaning |
|---|---|
| 'r' | open for reading (default) |
| 'w' | open for writing, truncating the file first |
| 'a' | open for writing, appending to the end of the file if it exists |
| 'b' | binary mode |
| 't' | text mode (default) |
| '+' | open a disk file for updating (reading and writing) |
| 'U' | universal newline mode (for backwards compatibility; should not be used in new code) |
The default mode is 'rt' (open for reading text). For binary random access, the mode 'w+b' opens and truncates the file to 0 bytes, while'r+b' opens the file without truncation.
Python distinguishes between files opened in binary and text modes, even when the underlying operating system doesn’t. Files opened in binary mode (including 'b' in the mode argument) return contents as bytesobjects without any decoding. In text mode (the default, or when 't' is included in the mode argument), the contents of the file are returned as strings, the bytes having been first decoded using a platform-dependent encoding or using the specified encoding if given.
buffering is an optional integer used to set the buffering policy. By default full buffering is on. Pass 0 to switch buffering off (only allowed in binary mode), 1 to set line buffering, and an integer > 1 for full buffering.
encoding is the name of the encoding used to decode or encode the file. This should only be used in text mode. The default encoding is platform dependent, but any encoding supported by Python can be used. See thecodecs module for the list of supported encodings.
errors is an optional string that specifies how encoding and decoding errors are to be handled. Pass 'strict' to raise a ValueErrorexception if there is an encoding error (the default of None has the same effect), or pass 'ignore' to ignore errors. (Note that ignoring encoding errors can lead to data loss.) 'replace' causes a replacement marker (such as '?') to be inserted where there is malformed data. When writing, 'xmlcharrefreplace' (replace with the appropriate XML character reference) or 'backslashreplace' (replace with backslashed escape sequences) can be used. Any other error handling name that has been registered with codecs.register_error() is also valid.
newline controls how universal newlines works (it only applies to text mode). It can be None, '', '\n', '\r', and '\r\n'. It works as follows:
- On input, if newline is None, universal newlines mode is enabled. Lines in the input can end in '\n', '\r', or '\r\n', and these are translated into '\n' before being returned to the caller. If it is'', universal newline mode is enabled, but line endings are returned to the caller untranslated. If it has any of the other legal values, input lines are only terminated by the given string, and the line ending is returned to the caller untranslated.
- On output, if newline is None, any '\n' characters written are translated to the system default line separator, os.linesep. If_newline_ is '', no translation takes place. If newline is any of the other legal values, any '\n' characters written are translated to the given string.
If closefd is False and a file descriptor rather than a filename was given, the underlying file descriptor will be kept open when the file is closed. If a filename is given closefd has no effect but must be True (the default).
The type of file object returned by the open() function depends on the mode. When open() is used to open a file in a text mode ('w', 'r', 'wt', 'rt', etc.), it returns aTextIOWrapper. When used to open a file in a binary mode, the returned class varies: in read binary mode, it returns aBufferedReader; in write binary and append binary modes, it returns a BufferedWriter, and in read/write mode, it returns a BufferedRandom.
It is also possible to use a string or bytearray as a file for both reading and writing. For strings StringIO can be used like a file opened in a text mode, and for bytearrays a BytesIO can be used like a file opened in a binary mode.