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.
Independently 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")
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.
io.DEFAULT_BUFFER_SIZE¶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.
io.open(file, mode='r', buffering=-1, encoding=None, errors=None, newline=None, closefd=True, opener=None)¶This is an alias for the builtin open() function.
io.BlockingIOError¶This is a compatibility alias for the builtin BlockingIOError
exception.
io.UnsupportedOperation¶An exception inheriting OSError and ValueError that is raised
when an unsupported operation is called on a stream.
It is also possible to use a str or bytes-like object as a
file for both reading and writing. For strings StringIO can be used
like a file opened in text mode. BytesIO can be used like a file
opened in binary mode. Both provide full read-write capabilities with random
access.
See also
syssys.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,
BufferedIOBaseprovides unoptimized implementations ofreadinto()andreadline().
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 deals with buffering on a raw byte stream
(RawIOBase). Its subclasses, BufferedWriter,
BufferedReader, and BufferedRWPair buffer streams that are
readable, writable, and both readable and writable. BufferedRandom
provides a buffered interface to random access streams. Another
BufferedIOBase subclass, BytesIO, is a stream of in-memory
bytes.
The TextIOBase ABC, another subclass of IOBase, deals with
streams whose bytes represent text, and handles encoding and decoding to and
from strings. TextIOWrapper, which extends it, 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 |
|---|---|---|---|
IOBase |
fileno, seek,
and truncate |
close, closed, __enter__,
__exit__, flush, isatty, __iter__,
__next__, readable, readline,
readlines, seekable, tell,
writable, and writelines |
|
RawIOBase |
IOBase |
readinto and
write |
Inherited IOBase methods, read,
and readall |
BufferedIOBase |
IOBase |
detach, read,
read1, and write |
Inherited IOBase methods, readinto |
TextIOBase |
IOBase |
detach, read,
readline, and
write |
Inherited IOBase methods, encoding,
errors, and newlines |
io.IOBase¶The abstract base class for all I/O classes, acting on streams of bytes. There is no public constructor.
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(), readinto(),
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. In some cases, such as
readinto(), a writable object such as bytearray
is required. 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:
close()¶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.
closed¶True if the stream is closed.
fileno()¶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()¶Flush the write buffers of the stream if applicable. This does nothing for read-only and non-blocking streams.
isatty()¶Return True if the stream is interactive (i.e., connected to
a terminal/tty device).
readline(size=-1)¶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.
readlines(hint=-1)¶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.
Note that it’s already possible to iterate on file objects using for
line in file: ... without calling file.readlines().
seek(offset[, whence])¶Change the stream position to the given byte offset. offset is
interpreted relative to the position indicated by whence. The default
value for whence is SEEK_SET. Values for whence are:
SEEK_SET or 0 – start of the stream (the default);
offset should be zero or positiveSEEK_CUR or 1 – current stream position; offset may
be negativeSEEK_END or 2 – end of the stream; offset is usually
negativeReturn the new absolute position.
New in version 3.1: The SEEK_* constants.
New 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.
seekable()¶Return True if the stream supports random access. If False,
seek(), tell() and truncate() will raise OSError.
tell()¶Return the current stream position.
truncate(size=None)¶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.
writable()¶Return True if the stream supports writing. If False,
write() and truncate() will raise OSError.
writelines(lines)¶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.
io.RawIOBase¶Base class for raw binary I/O. It inherits IOBase. There is no
public constructor.
Raw binary I/O typically provides low-level access to an underlying OS device or API, and does not try to encapsulate it in high-level primitives (this is left to Buffered I/O and Text I/O, described later in this page).
In addition to the attributes and methods from IOBase,
RawIOBase provides the following methods:
read(size=-1)¶Read up to size bytes from the object and return them. As a convenience,
if size is unspecified or -1, readall() is called. Otherwise,
only one system call is ever made. 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.
readall()¶Read and return all the bytes from the stream until EOF, using multiple calls to the stream if necessary.
readinto(b)¶Read bytes into a pre-allocated, writable
bytes-like object b, and return the
number of bytes read. If the object is in non-blocking mode and no bytes
are available, None is returned.
write(b)¶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.
io.BufferedIOBase¶Base class for binary streams that support some kind of buffering.
It inherits IOBase. There is no public constructor.
The main difference with RawIOBase is that methods read(),
readinto() and write() will try (respectively) to read as much
input as requested or to consume all given output, at the expense of
making perhaps more than one system call.
In addition, those methods can raise BlockingIOError if the
underlying raw stream is in non-blocking mode and cannot take or give
enough data; unlike their RawIOBase counterparts, they will
never return None.
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 methods and attribute in
addition to those from IOBase:
raw¶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.
detach()¶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.
New in version 3.1.
read(size=-1)¶Read and return up to size bytes. If the argument is omitted, None,
or negative, data is read and returned until EOF is reached. An empty
bytes object is returned if the stream is already at EOF.
If the argument is positive, and the underlying raw stream is not interactive, multiple raw reads may be issued to satisfy the byte count (unless EOF is reached first). But for interactive raw streams, at most one raw read will be issued, and a short result does not imply that EOF is imminent.
A BlockingIOError is raised if the underlying raw stream is in
non blocking-mode, and has no data available at the moment.
read1(size=-1)¶Read and return up to size bytes, with at most one call to the
underlying raw stream’s read() (or
readinto()) method. This can be useful if you are
implementing your own buffering on top of a BufferedIOBase
object.
readinto(b)¶Read bytes into a pre-allocated, writable bytes-like object b and return the number of bytes read.
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.
readinto1(b)¶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.
New in version 3.5.
write(b)¶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.
io.FileIO(name, mode='r', closefd=True, opener=None)¶FileIO represents an OS-level file containing bytes data.
It implements the RawIOBase interface (and therefore the
IOBase interface, too).
The name can be one of two things:
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.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.
The read() (when called with a positive argument), readinto()
and write() methods on this class will only make one system call.
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.
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.
In addition to the attributes and methods from IOBase and
RawIOBase, FileIO provides the following data
attributes:
mode¶The mode as given in the constructor.
name¶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.
io.BytesIO([initial_bytes])¶A stream implementation using an in-memory bytes buffer. It inherits
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.
BytesIO provides or overrides these methods in addition to those
from BufferedIOBase and IOBase:
getbuffer()¶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.
New in version 3.2.
io.BufferedReader(raw, buffer_size=DEFAULT_BUFFER_SIZE)¶A buffer providing higher-level access to a readable, sequential
RawIOBase object. It inherits 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:
peek([size])¶Return bytes from the stream without advancing the position. At most one single read on the raw stream is done to satisfy the call. The number of bytes returned may be less or more than requested.
read([size])¶Read and return size bytes, or if size is not given or negative, until EOF or if the read call would block in non-blocking mode.
read1(size)¶Read and return up to size bytes with only one call on the raw stream. If at least one byte is buffered, only buffered bytes are returned. Otherwise, one raw stream read call is made.
io.BufferedWriter(raw, buffer_size=DEFAULT_BUFFER_SIZE)¶A buffer providing higher-level access to a writeable, sequential
RawIOBase object. It inherits 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:
flush() is called;seek() is requested (for BufferedRandom objects);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:
flush()¶Force bytes held in the buffer into the raw stream. A
BlockingIOError should be raised if the raw stream blocks.
write(b)¶Write the bytes-like object, b, and return the
number of bytes written. When in non-blocking mode, a
BlockingIOError is raised if the buffer needs to be written out but
the raw stream blocks.
io.BufferedRandom(raw, buffer_size=DEFAULT_BUFFER_SIZE)¶A buffered interface to random access streams. It inherits
BufferedReader and BufferedWriter, and further supports
seek() and tell() functionality.
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.
io.BufferedRWPair(reader, writer, buffer_size=DEFAULT_BUFFER_SIZE)¶A buffered I/O object combining two unidirectional RawIOBase
objects – one readable, the other writeable – into a single bidirectional
endpoint. It inherits 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.
io.TextIOBase¶Base class for text streams. This class provides a character and line based
interface to stream I/O. There is no readinto() method because
Python’s character strings are immutable. It inherits IOBase.
There is no public constructor.
TextIOBase provides or overrides these data attributes and
methods in addition to those from IOBase:
encoding¶The name of the encoding used to decode the stream’s bytes into strings, and to encode strings into bytes.
errors¶The error setting of the decoder or encoder.
newlines¶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.
buffer¶The underlying binary buffer (a BufferedIOBase instance) that
TextIOBase deals with. This is not part of the
TextIOBase API and may not exist in some implementations.
detach()¶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.
New in version 3.1.
read(size)¶Read and return at most size characters from the stream as a single
str. If size is negative or None, reads until EOF.
readline(size=-1)¶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.
seek(offset[, whence])¶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.
New in version 3.1: The SEEK_* constants.
tell()¶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(s)¶Write the string s to the stream and return the number of characters written.
io.TextIOWrapper(buffer, encoding=None, errors=None, newline=None, line_buffering=False, write_through=False)¶A buffered text stream over a BufferedIOBase binary stream.
It inherits TextIOBase.
encoding gives the name of the encoding that the stream will be decoded or
encoded with. It defaults to
locale.getpreferredencoding(False).
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:
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.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.
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.
TextIOWrapper provides one attribute in addition to those of
TextIOBase and its parents:
line_buffering¶Whether line buffering is enabled.
io.StringIO(initial_value='', newline='\n')¶An in-memory stream for text I/O. 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.
The newline argument works like that of TextIOWrapper.
The default is to consider only \n characters as ends of lines and
to do no newline translation. If newline is set to None,
newlines are written as \n on all platforms, but universal
newline decoding is still performed when reading.
StringIO provides this method in addition to those from
TextIOBase and its parents:
getvalue()¶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()
io.IncrementalNewlineDecoder¶A helper codec that decodes newlines for universal newlines mode.
It inherits codecs.IncrementalDecoder.
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,
TextIOWrapper.tell() and TextIOWrapper.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 function print() as
well.