io — Core tools for working with streams¶Source code: Lib/io.py
The io module provides Python’s main facilities for dealing with various
types of I/O. There are three main types of I/O: text I/O, binary I/O
and raw I/O. These are generic categories, and various backing stores can
be used for each of them. A concrete object belonging to any of these
categories is called a file object. Other common terms are stream
and file-like object.
Independent of its category, each concrete stream object will also have various capabilities: it can be read-only, write-only, or read-write. It can also allow arbitrary random access (seeking forwards or backwards to any location), or only sequential access (for example in the case of a socket or pipe).
All streams are careful about the type of data you give to them. For example
giving a str object to the write() method of a binary stream
will raise a TypeError. So will giving a bytes object to the
write() method of a text stream.
Changed in version 3.3: Operations that used to raise IOError now raise OSError, since
IOError is now an alias of OSError.
Text I/O expects and produces str objects. This means that whenever
the backing store is natively made of bytes (such as in the case of a file),
encoding and decoding of data is made transparently as well as optional
translation of platform-specific newline characters.
The easiest way to create a text stream is with open(), optionally
specifying an encoding:
f = open("myfile.txt", "r", encoding="utf-8")
In-memory text streams are also available as StringIO objects:
f = io.StringIO("some initial text data")
Note
When working with a non-blocking stream, be aware that read operations on text I/O objects
might raise a BlockingIOError if the stream cannot perform the operation
immediately.
The text stream API is described in detail in the documentation of
TextIOBase.
Binary I/O (also called buffered I/O) expects
bytes-like objects and produces bytes
objects. No encoding, decoding, or newline translation is performed. This
category of streams can be used for all kinds of non-text data, and also when
manual control over the handling of text data is desired.
The easiest way to create a binary stream is with open() with 'b' in
the mode string:
f = open("myfile.jpg", "rb")
In-memory binary streams are also available as BytesIO objects:
f = io.BytesIO(b"some initial binary data: \x00\x01")
The binary stream API is described in detail in the docs of
BufferedIOBase.
Other library modules may provide additional ways to create text or binary
streams. See socket.socket.makefile() for example.
Raw I/O (also called unbuffered I/O) is generally used as a low-level building-block for binary and text streams; it is rarely useful to directly manipulate a raw stream from user code. Nevertheless, you can create a raw stream by opening a file in binary mode with buffering disabled:
f = open("myfile.jpg", "rb", buffering=0)
The raw stream API is described in detail in the docs of RawIOBase.
Warning
Raw I/O is a low-level interface and methods generally must have their return
values checked and be explicitly retried to ensure an operation completes.
For instance write() returns the number of bytes written
which may be less than the number of bytes provided (a partial write).
High-level I/O objects like Binary I/O and Text I/O implement
retry behavior.
The default encoding of TextIOWrapper and open() is
locale-specific (locale.getencoding()).
However, many developers forget to specify the encoding when opening text files encoded in UTF-8 (e.g. JSON, TOML, Markdown, etc…) since most Unix platforms use UTF-8 locale by default. This causes bugs because the locale encoding is not UTF-8 for most Windows users. For example:
# May not work on Windows when non-ASCII characters in the file.
with open("README.md") as f:
long_description = f.read()
Accordingly, it is highly recommended that you specify the encoding
explicitly when opening text files. If you want to use UTF-8, pass
encoding="utf-8". To use the current locale encoding,
encoding="locale" is supported since Python 3.10.
See also
Python UTF-8 Mode can be used to change the default encoding to UTF-8 from locale-specific encoding.
Python 3.15 will make Python UTF-8 Mode default.
Added in version 3.10: See PEP 597 for more details.
To find where the default locale encoding is used, you can enable
the -X warn_default_encoding command line option or set the
PYTHONWARNDEFAULTENCODING environment variable, which will
emit an EncodingWarning when the default encoding is used.
If you are providing an API that uses open() or
TextIOWrapper and passes encoding=None as a parameter, you
can use text_encoding() so that callers of the API will emit an
EncodingWarning if they don’t pass an encoding. However,
please consider using UTF-8 by default (i.e. encoding="utf-8") for
new APIs.
An int containing the default buffer size used by the module’s buffered I/O
classes. open() uses the file’s blksize (as obtained by
os.stat()) if possible.
This is an alias for the builtin open() function.
This function raises an auditing event open with
arguments path, mode and flags. The mode and flags
arguments may have been modified or inferred from the original call.
Opens the provided file with mode 'rb'. This function should be used
when the intent is to treat the contents as executable code.
path should be a str and an absolute path.
The behavior of this function may be overridden by an earlier call to the
PyFile_SetOpenCodeHook(). However, assuming that path is a
str and an absolute path, open_code(path) should always behave
the same as open(path, 'rb'). Overriding the behavior is intended for
additional validation or preprocessing of the file.
Added in version 3.8.
This is a helper function for callables that use open() or
TextIOWrapper and have an encoding=None parameter.
This function returns encoding if it is not None.
Otherwise, it returns "locale" or "utf-8" depending on
UTF-8 Mode.
This function emits an EncodingWarning if
sys.flags.warn_default_encoding is true and encoding
is None. stacklevel specifies where the warning is emitted.
For example:
def read_text(path, encoding=None):
encoding = io.text_encoding(encoding) # stacklevel=2
with open(path, encoding) as f:
return f.read()
In this example, an EncodingWarning is emitted for the caller of
read_text().
See Text Encoding for more information.
Added in version 3.10.
Changed in version 3.11: text_encoding() returns “utf-8” when UTF-8 mode is enabled and
encoding is None.
This is a compatibility alias for the builtin BlockingIOError
exception.
An exception inheriting OSError and ValueError that is raised
when an unsupported operation is called on a stream.
See also
syscontains the standard IO streams: sys.stdin, sys.stdout,
and sys.stderr.
The implementation of I/O streams is organized as a hierarchy of classes. First abstract base classes (ABCs), which are used to specify the various categories of streams, then concrete classes providing the standard stream implementations.
Note
The abstract base classes also provide default implementations of some
methods in order to help implementation of concrete stream classes. For
example, BufferedIOBase provides unoptimized implementations of
readinto() and readline().
At the top of the I/O hierarchy is the abstract base class IOBase. It
defines the basic interface to a stream. Note, however, that there is no
separation between reading and writing to streams; implementations are allowed
to raise UnsupportedOperation if they do not support a given operation.
The RawIOBase ABC extends IOBase. It deals with the reading
and writing of bytes to a stream. FileIO subclasses RawIOBase
to provide an interface to files in the machine’s file system.
The BufferedIOBase ABC extends IOBase. It deals with
buffering on a raw binary stream (RawIOBase). Its subclasses,
BufferedWriter, BufferedReader, and BufferedRWPair
buffer raw binary streams that are writable, readable, and both readable and writable,
respectively. BufferedRandom provides a buffered interface to seekable streams.
Another BufferedIOBase subclass, BytesIO, is a stream of
in-memory bytes.
The TextIOBase ABC extends IOBase. It deals with
streams whose bytes represent text, and handles encoding and decoding to and
from strings. TextIOWrapper, which extends TextIOBase, is a buffered text
interface to a buffered raw stream (BufferedIOBase). Finally,
StringIO is an in-memory stream for text.
Argument names are not part of the specification, and only the arguments of
open() are intended to be used as keyword arguments.
The following table summarizes the ABCs provided by the io module:
ABC |
Inherits |
Stub Methods |
Mixin Methods and Properties |
|---|---|---|---|
|
|
||
|
Inherited |
||
|
Inherited |
||
|
Inherited |
The abstract base class for all I/O classes.
This class provides empty abstract implementations for many methods that derived classes can override selectively; the default implementations represent a file that cannot be read, written or seeked.
Even though IOBase does not declare read()
or write() because their signatures will vary, implementations and
clients should consider those methods part of the interface. Also,
implementations may raise a ValueError (or UnsupportedOperation)
when operations they do not support are called.
The basic type used for binary data read from or written to a file is
bytes. Other bytes-like objects are
accepted as method arguments too. Text I/O classes work with str data.
Note that calling any method (even inquiries) on a closed stream is
undefined. Implementations may raise ValueError in this case.
IOBase (and its subclasses) supports the iterator protocol, meaning
that an IOBase object can be iterated over yielding the lines in a
stream. Lines are defined slightly differently depending on whether the
stream is a binary stream (yielding bytes), or a text stream (yielding
character strings). See readline() below.
IOBase is also a context manager and therefore supports the
with statement. In this example, file is closed after the
with statement’s suite is finished—even if an exception occurs:
with open('spam.txt', 'w') as file:
file.write('Spam and eggs!')
IOBase provides these data attributes and methods:
Flush and close this stream. This method has no effect if the file is
already closed. Once the file is closed, any operation on the file
(e.g. reading or writing) will raise a ValueError.
As a convenience, it is allowed to call this method more than once; only the first call, however, will have an effect.
True if the stream is closed.
Return the underlying file descriptor (an integer) of the stream if it
exists. An OSError is raised if the IO object does not use a file
descriptor.
Flush the write buffers of the stream if applicable. This does nothing for read-only and non-blocking streams.
Return True if the stream is interactive (i.e., connected to
a terminal/tty device).
Read and return one line from the stream. If size is specified, at most size bytes will be read.
The line terminator is always b'\n' for binary files; for text files,
the newline argument to open() can be used to select the line
terminator(s) recognized.
Read and return a list of lines from the stream. hint can be specified to control the number of lines read: no more lines will be read if the total size (in bytes/characters) of all lines so far exceeds hint.
hint values of 0 or less, as well as None, are treated as no
hint.
Note that it’s already possible to iterate on file objects using for
line in file: ... without calling file.readlines().
Change the stream position to the given byte offset, interpreted relative to the position indicated by whence, and return the new absolute position. Values for whence are:
os.SEEK_SET or 0 – start of the stream (the default);
offset should be zero or positive
os.SEEK_CUR or 1 – current stream position;
offset may be negative
os.SEEK_END or 2 – end of the stream;
offset is usually negative
Added in version 3.1: The SEEK_* constants.
Added in version 3.3: Some operating systems could support additional values, like
os.SEEK_HOLE or os.SEEK_DATA. The valid values
for a file could depend on it being open in text or binary mode.
Return True if the stream supports random access. If False,
seek(), tell() and truncate() will raise OSError.
Return the current stream position.
Resize the stream to the given size in bytes (or the current position if size is not specified). The current stream position isn’t changed. This resizing can extend or reduce the current file size. In case of extension, the contents of the new file area depend on the platform (on most systems, additional bytes are zero-filled). The new file size is returned.
Changed in version 3.5: Windows will now zero-fill files when extending.
Return True if the stream supports writing. If False,
write() and truncate() will raise OSError.
Write a list of lines to the stream. Line separators are not added, so it is usual for each of the lines provided to have a line separator at the end.
Base class for raw binary streams. It inherits from IOBase.
Raw binary streams typically provide low-level access to an underlying OS device or API, and do not try to encapsulate it in high-level primitives (this functionality is done at a higher-level in buffered binary streams and text streams, described later in this page).
RawIOBase provides these methods in addition to those from
IOBase:
Read up to size bytes from the object and return them. As a convenience, if size is unspecified or -1, all bytes until EOF are returned.
Attempts to make only one system call but will retry if interrupted and the signal handler does not raise an exception (see PEP 475 for the rationale). This means fewer than size bytes may be returned if the operating system call returns fewer than size bytes.
If 0 bytes are returned, and size was not 0, this indicates end of file.
If the object is in non-blocking mode and no bytes are available,
None is returned.
The default implementation defers to readall() and
readinto().
Read and return all the bytes from the stream until EOF, using multiple calls to the stream if necessary.
If 0 bytes are returned this indicates end of file. If the object is in
non-blocking mode and the underlying read() returns None
indicating no bytes are available, None is returned.
Read bytes into a pre-allocated, writable
bytes-like object b, and return the
number of bytes read. For example, b might be a bytearray.
If 0 is returned and len(b) is not 0, this indicates end of file. If
the object is in non-blocking mode and no bytes are available, None is
returned.
Write the given bytes-like object, b, to the
underlying raw stream, and return the number of
bytes written. This can be less than the length of b in
bytes, depending on specifics of the underlying raw
stream, and especially if it is in non-blocking mode. None is
returned if the raw stream is set not to block and no single byte could
be readily written to it. The caller may release or mutate b after
this method returns, so the implementation should only access b
during the method call.
Warning
This function does not ensure all bytes are written or an exception is thrown. Callers may implement that behavior by checking the return value and, if it is less than the length of b, looping with additional write calls until all unwritten bytes are written. High-level I/O objects like Binary I/O and Text I/O implement retry behavior.
Base class for binary streams that support some kind of buffering.
It inherits from IOBase.
The main difference with RawIOBase is that methods read(),
readinto() and write() will try (respectively) to read
as much input as requested or to emit all provided data.
In addition, if the underlying raw stream is in non-blocking mode, when the
system returns would block write() will raise BlockingIOError
with BlockingIOError.characters_written and read() will return
data read so far or None if no data is available.
Besides, the read() method does not have a default
implementation that defers to readinto().
A typical BufferedIOBase implementation should not inherit from a
RawIOBase implementation, but wrap one, like
BufferedWriter and BufferedReader do.
BufferedIOBase provides or overrides these data attributes and
methods in addition to those from IOBase:
The underlying raw stream (a RawIOBase instance) that
BufferedIOBase deals with. This is not part of the
BufferedIOBase API and may not exist on some implementations.
Separate the underlying raw stream from the buffer and return it.
After the raw stream has been detached, the buffer is in an unusable state.
Some buffers, like BytesIO, do not have the concept of a single
raw stream to return from this method. They raise
UnsupportedOperation.
Added in version 3.1.
Read and return up to size bytes. If the argument is omitted, None,
or negative read as much as possible.
Fewer bytes may be returned than requested. An empty bytes object
is returned if the stream is already at EOF. More than one read may be
made and calls may be retried if specific errors are encountered, see
os.read() and PEP 475 for more details. Less than size bytes
being returned does not imply that EOF is imminent.
When reading as much as possible the default implementation will use
raw.readall if available (which should implement
RawIOBase.readall()), otherwise will read in a loop until read
returns None, an empty bytes, or a non-retryable error. For
most streams this is to EOF, but for non-blocking streams more data may
become available.
Note
When the underlying raw stream is non-blocking, implementations may
either raise BlockingIOError or return None if no data is
available. io implementations return None.
Read and return up to size bytes, calling readinto()
which may retry if EINTR is encountered per
PEP 475. If size is -1 or not provided, the implementation will
choose an arbitrary value for size.
Note
When the underlying raw stream is non-blocking, implementations may
either raise BlockingIOError or return None if no data is
available. io implementations return None.
Read bytes into a pre-allocated, writable
bytes-like object b and return the number of bytes read.
For example, b might be a bytearray.
Like read(), multiple reads may be issued to the underlying raw
stream, unless the latter is interactive.
A BlockingIOError is raised if the underlying raw stream is in non
blocking-mode, and has no data available at the moment.
Read bytes into a pre-allocated, writable
bytes-like object b, using at most one call to
the underlying raw stream’s read() (or
readinto()) method. Return the number of bytes read.
A BlockingIOError is raised if the underlying raw stream is in non
blocking-mode, and has no data available at the moment.
Added in version 3.5.
Write the given bytes-like object, b, and return the number
of bytes written (always equal to the length of b in bytes, since if
the write fails an OSError will be raised). Depending on the
actual implementation, these bytes may be readily written to the
underlying stream, or held in a buffer for performance and latency
reasons.
When in non-blocking mode, a BlockingIOError is raised if the
data needed to be written to the raw stream but it couldn’t accept
all the data without blocking.
The caller may release or mutate b after this method returns, so the implementation should only access b during the method call.
A raw binary stream representing an OS-level file containing bytes data. It
inherits from RawIOBase and implements its low-level access design.
This means write() does not guarantee all bytes are written
and read() may read less bytes than requested even when more
bytes may be present in the underlying file. To get “write all” and
“read at least” behavior, use Binary I/O.
The name can be one of two things:
a character string or bytes object representing the path to the
file which will be opened. In this case closefd must be True (the default)
otherwise an error will be raised.
an integer representing the number of an existing OS-level file descriptor
to which the resulting FileIO object will give access. When the
FileIO object is closed this fd will be closed as well, unless closefd
is set to False.
The mode can be 'r', 'w', 'x' or 'a' for reading
(default), writing, exclusive creation or appending. The file will be
created if it doesn’t exist when opened for writing or appending; it will be
truncated when opened for writing. FileExistsError will be raised if
it already exists when opened for creating. Opening a file for creating
implies writing, so this mode behaves in a similar way to 'w'. Add a
'+' to the mode to allow simultaneous reading and writing.
A custom opener can be used by passing a callable as opener. The underlying
file descriptor for the file object is then obtained by calling opener with
(name, flags). opener must return an open file descriptor (passing
os.open as opener results in functionality similar to passing
None).
The newly created file is non-inheritable.
See the open() built-in function for examples on using the opener
parameter.
Warning
FileIO is a low-level I/O object and members, such as
read() and write(), need to have their
return values checked explicitly in a retry loop to implement “write all”
and “read at least” behavior. High-level I/O objects Binary I/O and
Text I/O implement retry behavior.
Changed in version 3.3: The opener parameter was added.
The 'x' mode was added.
Changed in version 3.4: The file is now non-inheritable.
FileIO provides these data attributes in addition to those from
RawIOBase and IOBase:
The mode as given in the constructor.
The file name. This is the file descriptor of the file when no name is given in the constructor.
Buffered I/O streams provide a higher-level interface to an I/O device than raw I/O does.
A binary stream using an in-memory bytes buffer. It inherits from
BufferedIOBase. The buffer is discarded when the
close() method is called.
The optional argument initial_bytes is a bytes-like object that contains initial data.
Methods may be used from multiple threads without external locking in free-threaded builds.
BytesIO provides or overrides these methods in addition to those
from BufferedIOBase and IOBase:
Return a readable and writable view over the contents of the buffer without copying them. Also, mutating the view will transparently update the contents of the buffer:
>>> b = io.BytesIO(b"abcdef")
>>> view = b.getbuffer()
>>> view[2:4] = b"56"
>>> b.getvalue()
b'ab56ef'
Note
As long as the view exists, the BytesIO object cannot be
resized or closed.
Added in version 3.2.
Return a copy of the buffer from the current position onwards without advancing the position.
If size is less than one or omitted, at most
DEFAULT_BUFFER_SIZE bytes are returned.
Otherwise, at most size bytes are returned.
Return an empty bytes object at EOF.
Added in version 3.16.0a0 (unreleased).
In BytesIO, this is the same as read().
Changed in version 3.7: The size argument is now optional.
In BytesIO, this is the same as readinto().
Added in version 3.5.
A buffered binary stream providing higher-level access to a readable, non
seekable RawIOBase raw binary stream. It inherits from
BufferedIOBase.
When reading data from this object, a larger amount of data may be requested from the underlying raw stream, and kept in an internal buffer. The buffered data can then be returned directly on subsequent reads.
The constructor creates a BufferedReader for the given readable
raw stream and buffer_size. If buffer_size is omitted,
DEFAULT_BUFFER_SIZE is used.
BufferedReader provides or overrides these methods in addition to
those from BufferedIOBase and IOBase:
Return bytes from the stream without advancing the position. The number of bytes returned may be less or more than requested. If the underlying raw stream is non-blocking and the operation would block, returns empty bytes.
In BufferedReader this is the same as io.BufferedIOBase.read()
In BufferedReader this is the same as io.BufferedIOBase.read1()
Changed in version 3.7: The size argument is now optional.
A buffered binary stream providing higher-level access to a writeable, non
seekable RawIOBase raw binary stream. It inherits from
BufferedIOBase.
When writing to this object, data is normally placed into an internal
buffer. The buffer will be written out to the underlying RawIOBase
object under various conditions, including:
when the buffer gets too small for all pending data;
when flush() is called;
when a seek() is requested (for BufferedRandom objects);
when the BufferedWriter object is closed or destroyed.
The constructor creates a BufferedWriter for the given writeable
raw stream. If the buffer_size is not given, it defaults to
DEFAULT_BUFFER_SIZE.
BufferedWriter provides or overrides these methods in addition to
those from BufferedIOBase and IOBase:
Force bytes held in the buffer into the raw stream. A
BlockingIOError should be raised if the raw stream blocks.
Write the bytes-like object, b, and return the
number of bytes written. When in non-blocking mode, a
BlockingIOError with BlockingIOError.characters_written set
is raised if the buffer needs to be written out but the raw stream blocks.
A buffered binary stream implementing BufferedIOBase interfaces
providing higher-level access to a seekable RawIOBase raw binary
stream.
The constructor creates a reader and writer for a seekable raw stream, given
in the first argument. If the buffer_size is omitted it defaults to
DEFAULT_BUFFER_SIZE.
BufferedRandom is capable of anything BufferedReader or
BufferedWriter can do. In addition, seek() and
tell() are guaranteed to be implemented.
A buffered binary stream providing higher-level access to two non seekable
RawIOBase raw binary streams—one readable, the other writeable.
It inherits from BufferedIOBase.
reader and writer are RawIOBase objects that are readable and
writeable respectively. If the buffer_size is omitted it defaults to
DEFAULT_BUFFER_SIZE.
BufferedRWPair implements all of BufferedIOBase's methods
except for detach(), which raises
UnsupportedOperation.
Warning
BufferedRWPair does not attempt to synchronize accesses to
its underlying raw streams. You should not pass it the same object
as reader and writer; use BufferedRandom instead.
Base class for text streams. This class provides a character and line based
interface to stream I/O. It inherits from IOBase.
TextIOBase provides or overrides these data attributes and
methods in addition to those from IOBase:
The name of the encoding used to decode the stream’s bytes into strings, and to encode strings into bytes.
The error setting of the decoder or encoder.
A string, a tuple of strings, or None, indicating the newlines
translated so far. Depending on the implementation and the initial
constructor flags, this may not be available.
The underlying binary buffer (a BufferedIOBase
or RawIOBase instance) that TextIOBase deals with.
This is not part of the TextIOBase API and may not exist
in some implementations.
Separate the underlying binary buffer from the TextIOBase and
return it.
After the underlying buffer has been detached, the TextIOBase is
in an unusable state.
Some TextIOBase implementations, like StringIO, may not
have the concept of an underlying buffer and calling this method will
raise UnsupportedOperation.
Added in version 3.1.
Read and return at most size characters from the stream as a single
str. If size is negative or None, reads until EOF.
Read until newline or EOF and return a single str. If the stream is
already at EOF, an empty string is returned.
If size is specified, at most size characters will be read.
Change the stream position to the given offset. Behaviour depends on
the whence parameter. The default value for whence is
SEEK_SET.
SEEK_SET or 0: seek from the start of the stream
(the default); offset must either be a number returned by
TextIOBase.tell(), or zero. Any other offset value
produces undefined behaviour.
SEEK_CUR or 1: “seek” to the current position;
offset must be zero, which is a no-operation (all other values
are unsupported).
SEEK_END or 2: seek to the end of the stream;
offset must be zero (all other values are unsupported).
Return the new absolute position as an opaque number.
Added in version 3.1: The SEEK_* constants.
Return the current stream position as an opaque number. The number does not usually represent a number of bytes in the underlying binary storage.
Write the string s to the stream and return the number of characters written.
A buffered text stream providing higher-level access to a
BufferedIOBase buffered binary stream. It inherits from
TextIOBase.
encoding gives the name of the encoding that the stream will be decoded or
encoded with. In UTF-8 Mode, this defaults to UTF-8.
Otherwise, it defaults to locale.getencoding().
encoding="locale" can be used to specify the current locale’s encoding
explicitly. See Text Encoding for more information.
errors is an optional string that specifies how encoding and decoding
errors are to be handled. 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.
'backslashreplace' causes malformed data to be replaced by a
backslashed escape sequence. When writing, 'xmlcharrefreplace'
(replace with the appropriate XML character reference) or 'namereplace'
(replace with \N{...} escape sequences) can be used. Any other error
handling name that has been registered with
codecs.register_error() is also valid.
newline controls how line endings are handled. It can be None,
'', '\n', '\r', and '\r\n'. It works as follows:
When reading input from the stream, 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 newline is '', universal
newlines mode is enabled, but line endings are returned to the caller
untranslated. If newline 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.
When writing output to the stream, if newline is None, any '\n'
characters written are translated to the system default line separator,
os.linesep. If newline is '' or '\n', no translation
takes place. If newline is any of the other legal values, any '\n'
characters written are translated to the given string.
If line_buffering is True, flush() is implied when a call to
write contains a newline character or a carriage return.
If write_through is True, calls to write() are guaranteed
not to be buffered: any data written on the TextIOWrapper
object is immediately handled to its underlying binary buffer.
Changed in version 3.3: The write_through argument has been added.
Changed in version 3.3: The default encoding is now locale.getpreferredencoding(False)
instead of locale.getpreferredencoding(). Don’t change temporary the
locale encoding using locale.setlocale(), use the current locale
encoding instead of the user preferred encoding.
Changed in version 3.10: The encoding argument now supports the "locale" dummy encoding name.
Note
When the underlying raw stream is non-blocking, a BlockingIOError
may be raised if a read operation cannot be completed immediately.
TextIOWrapper provides these data attributes and methods in
addition to those from TextIOBase and IOBase:
Whether line buffering is enabled.
Whether writes are passed immediately to the underlying binary buffer.
Added in version 3.7.
Reconfigure this text stream using new settings for encoding, errors, newline, line_buffering and write_through.
Parameters not specified keep current settings, except
errors='strict' is used when encoding is specified but
errors is not specified.
It is not possible to change the encoding or newline if some data has already been read from the stream. On the other hand, changing encoding after write is possible.
This method does an implicit stream flush before setting the new parameters.
Added in version 3.7.
Changed in version 3.11: The method supports encoding="locale" option.
Set the stream position.
Return the new stream position as an int.
Four operations are supported, given by the following argument combinations:
seek(0, SEEK_SET): Rewind to the start of the stream.
seek(cookie, SEEK_SET): Restore a previous position;
cookie must be a number returned by tell().
seek(0, SEEK_END): Fast-forward to the end of the stream.
seek(0, SEEK_CUR): Leave the current stream position unchanged.
Any other argument combinations are invalid, and may raise exceptions.
See also
os.SEEK_SET, os.SEEK_CUR, and os.SEEK_END.
A text stream using an in-memory text buffer. It inherits from
TextIOBase.
The text buffer is discarded when the close() method is
called.
The initial value of the buffer can be set by providing initial_value.
If newline translation is enabled, newlines will be encoded as if by
write(). The stream is positioned at the start of the
buffer which emulates opening an existing file in a w+ mode, making it
ready for an immediate write from the beginning or for a write that
would overwrite the initial value. To emulate opening a file in an a+
mode ready for appending, use f.seek(0, io.SEEK_END) to reposition the
stream at the end of the buffer.
The newline argument works like that of TextIOWrapper,
except that when writing output to the stream, if newline is None,
newlines are written as \n on all platforms.
StringIO provides this method in addition to those from
TextIOBase and IOBase:
Return a str containing the entire contents of the buffer.
Newlines are decoded as if by read(), although
the stream position is not changed.
Example usage:
import io
output = io.StringIO()
output.write('First line.\n')
print('Second line.', file=output)
# Retrieve file contents -- this will be
# 'First line.\nSecond line.\n'
contents = output.getvalue()
# Close object and discard memory buffer --
# .getvalue() will now raise an exception.
output.close()
A helper codec that decodes newlines for universal newlines mode.
It inherits from codecs.IncrementalDecoder.
The following protocols can be used for annotating function and method
arguments for simple stream reading or writing operations. They are decorated
with @typing.runtime_checkable.
Generic protocol for reading from a file or other input stream. T will
usually be str or bytes, but can be any type that is
read from the stream.
Added in version 3.14.
Read data from the input stream and return it. If size is specified, it should be an integer, and at most size items (bytes/characters) will be read.
For example:
def read_it(reader: Reader[str]):
data = reader.read(11)
assert isinstance(data, str)
Generic protocol for writing to a file or other output stream. T will
usually be str or bytes, but can be any type that can be
written to the stream.
Added in version 3.14.
Write data to the output stream and return the number of items (bytes/characters) written.
For example:
def write_binary(writer: Writer[bytes]):
writer.write(b"Hello world!\n")
See ABCs and Protocols for working with I/O for other I/O related protocols and classes that can be used for static type checking.
This section discusses the performance of the provided concrete I/O implementations.
By reading and writing only large chunks of data even when the user asks for a single byte, buffered I/O hides any inefficiency in calling and executing the operating system’s unbuffered I/O routines. The gain depends on the OS and the kind of I/O which is performed. For example, on some modern OSes such as Linux, unbuffered disk I/O can be as fast as buffered I/O. The bottom line, however, is that buffered I/O offers predictable performance regardless of the platform and the backing device. Therefore, it is almost always preferable to use buffered I/O rather than unbuffered I/O for binary data.
Text I/O over a binary storage (such as a file) is significantly slower than
binary I/O over the same storage, because it requires conversions between
unicode and binary data using a character codec. This can become noticeable
handling huge amounts of text data like large log files. Also,
tell() and seek() are both quite slow
due to the reconstruction algorithm used.
StringIO, however, is a native in-memory unicode container and will
exhibit similar speed to BytesIO.
FileIO objects are thread-safe to the extent that the operating system
calls (such as read(2) under Unix) they wrap are thread-safe too.
Binary buffered objects (instances of BufferedReader,
BufferedWriter, BufferedRandom and BufferedRWPair)
protect their internal structures using a lock; it is therefore safe to call
them from multiple threads at once.
TextIOWrapper objects are not thread-safe.
Binary buffered objects (instances of BufferedReader,
BufferedWriter, BufferedRandom and BufferedRWPair)
are not reentrant. While reentrant calls will not happen in normal situations,
they can arise from doing I/O in a signal handler. If a thread tries to
re-enter a buffered object which it is already accessing, a RuntimeError
is raised. Note this doesn’t prohibit a different thread from entering the
buffered object.
The above implicitly extends to text files, since the open() function
will wrap a buffered object inside a TextIOWrapper. This includes
standard streams and therefore affects the built-in print() function as
well.