| Python 3.6.5 Documentation > "io" — Core tools for working with streams
"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*.
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
--------
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.
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()", "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).
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])
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. 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=-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.
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()
In "BytesIO", this is the same as "read()".
readinto1()
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.
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", 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.
Text I/O
--------
class 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.
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.
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.
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.
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