text: 16.2. io — Core tools for working with streams
  16.2.1. 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.
  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.
  16.2.1.1. 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.
  16.2.1.2. Binary I/O
  Binary I/O (also called buffered I/O) expects 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.
  16.2.1.3. 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.
  16.2.2. High-level Module Interface
  - io.DEFAULT_BUFFER_SIZE, opener=None)
  This is an alias for the builtin open() function.
  - 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.
  16.2.2.1. In-memory streams
  - sys
  - contains the standard IO streams: sys.stdin, sys.stdout, and sys.stderr.
  16.2.3. Class hierarchy.
  The following table summarizes the ABCs provided by the io module:
  16.2.3.1. I/O Base Classes
  - class io.IOBase ValueError (or UnsupportedOperation) 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..
  - 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)])
  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..
  - 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)()
  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.
  - class io.RawIOBase(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 up to len(b) bytes into bytearray 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).
  - 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().
  New in version 3.1.
  - read(size=-1)
  Read and return up to size 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() method. This can be useful if you are implementing your own buffering on top of a BufferedIOBase object.
  - readinto(b)
  Read up to len(b) bytes into bytearray 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.
  - write(b)
  Write the given bytes or bytearray object, b and return the number of bytes written (never less than len(b), 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.
  16.2.3.2. 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;
  - an integer representing the number of an existing OS-level file descriptor to which the resulting FileIO object will give access.
  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.
  16.2.3.3. 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 argument initial_bytes contains optional initial bytes 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'
  New in version 3.2.
  - class io.BufferedReader(raw, buffer_size=DEFAULT_BUFFER_SIZE)
  A buffer providing higher-level access to a readable, sequential RawIOBase object. 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.
  - 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 or bytearray 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, 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.
  - 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.
  16.2.3.4. Text I/O
  - class io.TextIOBase:
  - 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.
  - 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). 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.
  - 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. 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.
  16.2.4. Performance
  This section discusses the performance of the provided concrete I/O implementations.
  16.2.4.1. almost always preferable to use buffered I/O rather than unbuffered I/O for binary data.
  16.2.4, 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.
  16.2.4.3. Multi-threading
  FileIO objects are thread-safe to the extent that the operating system calls (such as read(2) under Unix) they wrap.
  16.2.4.4. 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.