Open file and return a corresponding stream. If the file cannot be opened,
an IOError is raised.
file is either a string giving the pathname (absolute or
relative to the current working directory) of the file to be opened or
an integer file descriptor of the file to be wrapped. (If a file descriptor
is given, 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 newlines 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 bytes
objects 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
unicode 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.
Pass 0 to switch buffering off (only allowed in binary mode), 1 to select
line buffering (only usable in text mode), and an integer > 1 to indicate
the size of a fixed-size chunk buffer. When no buffering argument is
given, the default buffering policy works as follows:
- Binary files are buffered in fixed-size chunks; the size of the buffer
is chosen using a heuristic trying to determine the underlying device’s
“block size” and falling back on
DEFAULT_BUFFER_SIZE.
On many systems, the buffer will typically be 4096 or 8192 bytes long.
- “Interactive” text files (files for which
isatty() returns True)
use line buffering. Other text files use the policy described above
for binary files.
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 (whatever locale.getpreferredencoding() returns), but any
encoding supported by Python can be used. See the codecs module for
the list of supported encodings.
errors is an optional string that specifies how encoding and decoding
errors are to be handled—this cannot be used in binary mode. Pass
'strict' to raise a ValueError exception 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 newlines 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 and 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 a subclass of
TextIOBase (specifically TextIOWrapper). When used to open
a file in a binary mode with buffering, the returned class is a subclass of
BufferedIOBase. The exact class varies: in read binary mode, it
returns a BufferedReader; in write binary and append binary modes,
it returns a BufferedWriter, and in read/write mode, it returns a
BufferedRandom. When buffering is disabled, the raw stream, a
subclass of RawIOBase, FileIO, is returned.
It is also possible to use an unicode or bytes string
as a file for both reading and writing. For unicode strings
StringIO can be used like a file opened in text mode,
and for bytes a BytesIO can be used like a
file opened in a binary mode.