"io" — Core tools for working with streams
******************************************

**Source code:** Lib/io.py

======================================================================


Overview
========

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
--------

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".


High-level Module Interface
===========================

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.

   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.

io.open_code(path)

   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 an absolute path.

   The behavior of this function may be overridden by an earlier call
   to the "PyFile_SetOpenCodeHook()", however, it should always be
   considered interchangeable with "open(path, 'rb')". Overriding the
   behavior is intended for additional validation or preprocessing of
   the file.

   New in version 3.8.

exception io.BlockingIOError

   This is a compatibility alias for the builtin "BlockingIOError"
   exception.

exception io.UnsupportedOperation

   An exception inheriting "OSError" and "ValueError" that is raised
   when an unsupported operation is called on a stream.


In-memory streams
-----------------

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:

  "sys"
     contains the standard IO streams: "sys.stdin", "sys.stdout", and
     "sys.stderr".


Class hierarchy
===============

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 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    | "close", "closed", "__enter__", "__exit__",        |
|                           |                    | "truncate"               | "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",        | Inherited "IOBase" methods, "readinto", and        |
|                           |                    | "read1", and "write"     | "readinto1"                                        |
+---------------------------+--------------------+--------------------------+----------------------------------------------------+
| "TextIOBase"              | "IOBase"           | "detach", "read",        | Inherited "IOBase" methods, "encoding", "errors",  |
|                           |                    | "readline", and "write"  | and "newlines"                                     |
+---------------------------+--------------------+--------------------------+----------------------------------------------------+


I/O Base Classes
----------------

class 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()" 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*.

      Note that it’s already possible to iterate on file objects using
      "for line in file: ..." without calling "file.readlines()".

   seek(offset, whence=SEEK_SET)

      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 positive

      * "SEEK_CUR" or "1" – current stream position; *offset* may be
        negative

      * "SEEK_END" or "2" – end of the stream; *offset* is usually
        negative

      Return 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.

   __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 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, 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 "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])

      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.

      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.


Raw File I/O
------------

class 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:

   * 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.

   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 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])

   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.

   getvalue()

      Return "bytes" containing the entire contents of the buffer.

   read1([size])

      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()".

      New in version 3.5.

class 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.

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

class 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:

   * 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 interface to random access streams.  It inherits
   "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 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.


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 "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=-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.

      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.

class 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:

   * 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 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.

   * 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.

   "TextIOWrapper" provides these members in addition to those of
   "TextIOBase" and its parents:

   line_buffering

      Whether line buffering is enabled.

   write_through

      Whether writes are passed immediately to the underlying binary
      buffer.

      New in version 3.7.

   reconfigure(*[, encoding][, errors][, newline][,                      line_buffering][, write_through])

      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.

      New in version 3.7.

class 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()

class io.IncrementalNewlineDecoder

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


Performance
===========

This section discusses the performance of the provided concrete I/O
implementations.


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, "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".


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 function
"print()" as well.
