io — Core tools for working with streams¶

Text I/O¶

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¶

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¶

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.

Opt-in EncodingWarning¶

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.

I/O Base Classes¶

class io.IOBase¶

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:

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).

readable()¶

Return True if the stream can be read from. If False, read() will raise OSError.

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.

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().

seek(offset, whence=os.SEEK_SET, /)¶

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.

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.

__del__()¶

Prepare for object destruction. IOBase provides a default implementation of this method that calls the instance’s close() method.

class io.RawIOBase¶

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(size=-1, /)¶

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. 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.

The default implementation defers to readall() and readinto().

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. For example, b might be a bytearray. 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.

class io.BufferedIOBase¶

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 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 data attributes and methods 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.

Added 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.

If size is -1 (the default), an arbitrary number of bytes are returned (more than zero unless EOF is reached).

readinto(b, /)¶

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.

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.

Added 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.

Raw File I/O¶

class io.FileIO(name, mode='r', closefd=True, opener=None)¶

A raw binary stream representing an OS-level file containing bytes data. It inherits from RawIOBase.

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.

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.

FileIO provides these data attributes in addition to those from RawIOBase and IOBase:

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 Streams¶

Buffered I/O streams provide a higher-level interface to an I/O device than raw I/O does.

class io.BytesIO(initial_bytes=b'')¶

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.

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.

Added in version 3.2.

getvalue()¶

Return bytes containing the entire contents of the buffer.

read1(size=-1, /)¶

In BytesIO, this is the same as read().

Changed in version 3.7: The size argument is now optional.

readinto1(b, /)¶

In BytesIO, this is the same as readinto().

Added in version 3.5.

class io.BufferedReader(raw, buffer_size=DEFAULT_BUFFER_SIZE)¶

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:

peek(size=0, /)¶

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=-1, /)¶

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.

Note

When the underlying raw stream is non-blocking, a BlockingIOError may be raised if a read operation cannot be completed immediately.

read1(size=-1, /)¶

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.

Changed in version 3.7: The size argument is now optional.

Note

When the underlying raw stream is non-blocking, a BlockingIOError may be raised if a read operation cannot be completed immediately.

class io.BufferedWriter(raw, buffer_size=DEFAULT_BUFFER_SIZE)¶

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:

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.

class io.BufferedRandom(raw, buffer_size=DEFAULT_BUFFER_SIZE)¶

A buffered binary stream providing higher-level access to a seekable RawIOBase raw binary stream. It inherits from BufferedReader and BufferedWriter.

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.

class io.BufferedRWPair(reader, writer, buffer_size=DEFAULT_BUFFER_SIZE, /)¶

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.

Text I/O¶

class io.TextIOBase¶

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:

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.

Added in version 3.1.

read(size=-1, /)¶

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=SEEK_SET, /)¶

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.

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.

class io.TextIOWrapper(buffer, encoding=None, errors=None, newline=None, line_buffering=False, write_through=False)¶

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. 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:

line_buffering¶

Whether line buffering is enabled.

write_through¶

Whether writes are passed immediately to the underlying binary buffer.

Added in version 3.7.

reconfigure(*, encoding=None, errors=None, newline=None, line_buffering=None, write_through=None)¶

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.

seek(cookie, whence=os.SEEK_SET, /)¶

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.

tell()¶

Return the stream position as an opaque number. The return value of tell() can be given as input to seek(), to restore a previous stream position.

class io.StringIO(initial_value='', newline='\n')¶

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:

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()

class io.IncrementalNewlineDecoder¶

A helper codec that decodes newlines for universal newlines mode. It inherits from codecs.IncrementalDecoder.

Binary I/O¶

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¶

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.

Multi-threading¶

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.

Reentrancy¶

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.