io— Core tools for working with streams

Source code:Lib/io.py

---

Overview

Theiomodule 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 andraw 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 afile object.Other common terms arestream andfile-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 astrobject to thewrite()method of a binary stream will raise aTypeError.So will giving abytesobject to the write()method of a text stream.

Changed in version 3.3:Operations that used to raiseIOErrornow raiseOSError,since IOErroris now an alias ofOSError.

Text I/O

Text I/O expects and producesstrobjects. 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 withopen(),optionally specifying an encoding:

f=open("myfile.txt","r",encoding="utf-8")

In-memory text streams are also available asStringIOobjects:

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 calledbuffered I/O) expects bytes-like objectsand producesbytes 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 withopen()with'b'in the mode string:

f=open("myfile.jpg","rb")

In-memory binary streams are also available asBytesIOobjects:

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. Seesocket.socket.makefile()for example.

Raw I/O

Raw I/O (also calledunbuffered 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 ofRawIOBase.

Text Encoding

The default encoding ofTextIOWrapperandopen()is locale-specific (locale.getencoding()).

However, many developers forget to specify the encoding when opening text files encoded in UTF-8 (e.g. JSON, TOML, Markdown, etc…) since most Unix platforms use UTF-8 locale by default. This causes bugs because the locale encoding is not UTF-8 for most Windows users. For example:

# May not work on Windows when non-ASCII characters in the file.
withopen("README.md")asf:
long_description=f.read()

Accordingly, it is highly recommended that you specify the encoding explicitly when opening text files. If you want to use UTF-8, pass encoding= "utf-8".To use the current locale encoding, encoding= "locale"is supported since Python 3.10.

See also

Python UTF-8 Mode

Python UTF-8 Mode can be used to change the default encoding to UTF-8 from locale-specific encoding.

PEP 686

Python 3.15 will makePython UTF-8 Modedefault.

Opt-in EncodingWarning

Added in version 3.10:SeePEP 597for more details.

To find where the default locale encoding is used, you can enable the-Xwarn_default_encodingcommand line option or set the PYTHONWARNDEFAULTENCODINGenvironment variable, which will emit anEncodingWarningwhen the default encoding is used.

If you are providing an API that usesopen()or TextIOWrapperand passesencoding=Noneas a parameter, you can usetext_encoding()so that callers of the API will emit an EncodingWarningif they don’t pass anencoding.However, please consider using UTF-8 by default (i.e.encoding= "utf-8") for new APIs.

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 builtinopen()function.

This function raises anauditing eventopenwith argumentspath,modeandflags.Themodeandflags 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.

pathshould be astrand an absolute path.

The behavior of this function may be overridden by an earlier call to the PyFile_SetOpenCodeHook().However, assuming thatpathis a strand an absolute path,open_code(path)should always behave the same asopen(path,'rb').Overriding the behavior is intended for additional validation or preprocessing of the file.

Added in version 3.8.

io.text_encoding(encoding,stacklevel=2,/)

This is a helper function for callables that useopen()or TextIOWrapperand have anencoding=Noneparameter.

This function returnsencodingif it is notNone. Otherwise, it returns"locale"or"utf-8"depending on UTF-8 Mode.

This function emits anEncodingWarningif sys.flags.warn_default_encodingis true andencoding isNone.stacklevelspecifies where the warning is emitted. For example:

defread_text(path,encoding=None):
encoding=io.text_encoding(encoding)# stacklevel=2
withopen(path,encoding)asf:
returnf.read()

In this example, anEncodingWarningis emitted for the caller of read_text().

SeeText Encodingfor more information.

Added in version 3.10.

Changed in version 3.11:text_encoding()returns “utf-8” when UTF-8 mode is enabled and encodingisNone.

exceptionio.BlockingIOError

This is a compatibility alias for the builtinBlockingIOError exception.

exceptionio.UnsupportedOperation

An exception inheritingOSErrorandValueErrorthat is raised when an unsupported operation is called on a stream.

See also

sys

contains the standard IO streams:sys.stdin,sys.stdout, andsys.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,BufferedIOBaseprovides unoptimized implementations of readinto()andreadline().

At the top of the I/O hierarchy is the abstract base classIOBase.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 raiseUnsupportedOperationif they do not support a given operation.

TheRawIOBaseABC extendsIOBase.It deals with the reading and writing of bytes to a stream.FileIOsubclassesRawIOBase to provide an interface to files in the machine’s file system.

TheBufferedIOBaseABC extendsIOBase.It deals with buffering on a raw binary stream (RawIOBase). Its subclasses, BufferedWriter,BufferedReader,andBufferedRWPair buffer raw binary streams that are writable, readable, and both readable and writable, respectively.BufferedRandomprovides a buffered interface to seekable streams. AnotherBufferedIOBasesubclass,BytesIO,is a stream of in-memory bytes.

TheTextIOBaseABC extendsIOBase.It deals with streams whose bytes represent text, and handles encoding and decoding to and from strings.TextIOWrapper,which extendsTextIOBase,is a buffered text interface to a buffered raw stream (BufferedIOBase). Finally, StringIOis 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 theiomodule:

ABC	Inherits	Stub Methods	Mixin Methods and Properties
IOBase		fileno,seek, andtruncate	close,closed,__enter__, __exit__,flush,isatty,__iter__, __next__,readable,readline, readlines,seekable,tell, writable,andwritelines
RawIOBase	IOBase	readintoand write	InheritedIOBasemethods,read, andreadall
BufferedIOBase	IOBase	detach,read, read1,andwrite	InheritedIOBasemethods,readinto, andreadinto1
TextIOBase	IOBase	detach,read, readline,and write	InheritedIOBasemethods,encoding, errors,andnewlines

I/O Base Classes

classio.IOBase

The abstract base class for all I/O classes.

This class provides empty abstract implementations for many methods that derived classes can override selectively; the default implementations represent a file that cannot be read, written or seeked.

Even thoughIOBasedoes not declareread() orwrite()because their signatures will vary, implementations and clients should consider those methods part of the interface. Also, implementations may raise aValueError(orUnsupportedOperation) when operations they do not support are called.

The basic type used for binary data read from or written to a file is bytes.Otherbytes-like objectsare accepted as method arguments too. Text I/O classes work withstrdata.

Note that calling any method (even inquiries) on a closed stream is undefined. Implementations may raiseValueErrorin this case.

IOBase(and its subclasses) supports the iterator protocol, meaning that anIOBaseobject 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). Seereadline()below.

IOBaseis also a context manager and therefore supports the withstatement. In this example,fileis closed after the withstatement’s suite is finished—even if an exception occurs:

withopen('spam.txt','w')asfile:
file.write('Spam and eggs!')

IOBaseprovides 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 aValueError.

As a convenience, it is allowed to call this method more than once; only the first call, however, will have an effect.

closed

Trueif the stream is closed.

fileno()

Return the underlying file descriptor (an integer) of the stream if it exists. AnOSErroris 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()

ReturnTrueif the stream is interactive (i.e., connected to a terminal/tty device).

readable()

ReturnTrueif the stream can be read from. IfFalse,read()will raiseOSError.

readline(size=-1,/)

Read and return one line from the stream. Ifsizeis specified, at mostsizebytes will be read.

The line terminator is alwaysb'\n'for binary files; for text files, thenewlineargument toopen()can be used to select the line terminator(s) recognized.

readlines(hint=-1,/)

Read and return a list of lines from the stream.hintcan 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 exceedshint.

hintvalues of0or less, as well asNone,are treated as no hint.

Note that it’s already possible to iterate on file objects usingfor lineinfile:...without callingfile.readlines().

seek(offset,whence=os.SEEK_SET,/)

Change the stream position to the given byteoffset, interpreted relative to the position indicated bywhence, and return the new absolute position. Values forwhenceare:

• os.SEEK_SETor0– start of the stream (the default); offsetshould be zero or positive

• os.SEEK_CURor1– current stream position; offsetmay be negative

• os.SEEK_ENDor2– end of the stream; offsetis usually negative

Added in version 3.1:TheSEEK_*constants.

Added in version 3.3:Some operating systems could support additional values, like os.SEEK_HOLEoros.SEEK_DATA.The valid values for a file could depend on it being open in text or binary mode.

seekable()

ReturnTrueif the stream supports random access. IfFalse, seek(),tell()andtruncate()will raiseOSError.

tell()

Return the current stream position.

truncate(size=None,/)

Resize the stream to the givensizein bytes (or the current position ifsizeis 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()

ReturnTrueif the stream supports writing. IfFalse, write()andtruncate()will raiseOSError.

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.IOBaseprovides a default implementation of this method that calls the instance’s close()method.

classio.RawIOBase

Base class for raw binary streams. It inherits fromIOBase.

Raw binary streams typically provide low-level access to an underlying OS device or API, and do not try to encapsulate it in high-level primitives (this functionality is done at a higher-level in buffered binary streams and text streams, described later in this page).

RawIOBaseprovides these methods in addition to those from IOBase:

read(size=-1,/)

Read up tosizebytes from the object and return them. As a convenience, ifsizeis unspecified or -1, all bytes until EOF are returned. Otherwise, only one system call is ever made. Fewer thansizebytes may be returned if the operating system call returns fewer thansizebytes.

If 0 bytes are returned, andsizewas not 0, this indicates end of file. If the object is in non-blocking mode and no bytes are available, Noneis returned.

The default implementation defers toreadall()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 objectb,and return the number of bytes read. For example,bmight be abytearray. If the object is in non-blocking mode and no bytes are available,Noneis returned.

write(b,/)

Write the givenbytes-like object,b,to the underlying raw stream, and return the number of bytes written. This can be less than the length ofbin bytes, depending on specifics of the underlying raw stream, and especially if it is in non-blocking mode.Noneis 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 mutatebafter this method returns, so the implementation should only accessb during the method call.

classio.BufferedIOBase

Base class for binary streams that support some kind of buffering. It inherits fromIOBase.

The main difference withRawIOBaseis that methodsread(), readinto()andwrite()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 raiseBlockingIOErrorif the underlying raw stream is in non-blocking mode and cannot take or give enough data; unlike theirRawIOBasecounterparts, they will never returnNone.

Besides, theread()method does not have a default implementation that defers toreadinto().

A typicalBufferedIOBaseimplementation should not inherit from a RawIOBaseimplementation, but wrap one, like BufferedWriterandBufferedReaderdo.

BufferedIOBaseprovides or overrides these data attributes and methods in addition to those fromIOBase:

raw

The underlying raw stream (aRawIOBaseinstance) that BufferedIOBasedeals with. This is not part of the BufferedIOBaseAPI 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, likeBytesIO,do not have the concept of a single raw stream to return from this method. They raise UnsupportedOperation.

Added in version 3.1.

read(size=-1,/)

Read and return up tosizebytes. If the argument is omitted,None, or negative, data is read and returned until EOF is reached. An empty bytesobject 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.

ABlockingIOErroris 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 tosizebytes, with at most one call to the underlying raw stream’sread()(or readinto()) method. This can be useful if you are implementing your own buffering on top of aBufferedIOBase object.

Ifsizeis-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 objectband return the number of bytes read. For example,bmight be abytearray.

Likeread(),multiple reads may be issued to the underlying raw stream, unless the latter is interactive.

ABlockingIOErroris 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 objectb,using at most one call to the underlying raw stream’sread()(or readinto()) method. Return the number of bytes read.

ABlockingIOErroris raised if the underlying raw stream is in non blocking-mode, and has no data available at the moment.

Added in version 3.5.

write(b,/)

Write the givenbytes-like object,b,and return the number of bytes written (always equal to the length ofbin bytes, since if the write fails anOSErrorwill 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, aBlockingIOErroris 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 mutatebafter this method returns, so the implementation should only accessbduring the method call.

Raw File I/O

classio.FileIO(name,mode='r',closefd=True,opener=None)

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

Thenamecan be one of two things:

• a character string orbytesobject representing the path to the file which will be opened. In this case closefd must beTrue(the default) otherwise an error will be raised.

• an integer representing the number of an existing OS-level file descriptor to which the resultingFileIOobject will give access. When the FileIO object is closed this fd will be closed as well, unlessclosefd is set toFalse.

Themodecan 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.FileExistsErrorwill 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.

Theread()(when called with a positive argument), readinto()andwrite()methods on this class will only make one system call.

A custom opener can be used by passing a callable asopener.The underlying file descriptor for the file object is then obtained by callingopenerwith (name,flags).openermust return an open file descriptor (passing os.openasopenerresults in functionality similar to passing None).

The newly created file isnon-inheritable.

See theopen()built-in function for examples on using theopener parameter.

Changed in version 3.3:Theopenerparameter was added. The'x'mode was added.

Changed in version 3.4:The file is now non-inheritable.

FileIOprovides these data attributes in addition to those from RawIOBaseandIOBase:

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.

classio.BytesIO(initial_bytes=b'')

A binary stream using an in-memory bytes buffer. It inherits from BufferedIOBase.The buffer is discarded when the close()method is called.

The optional argumentinitial_bytesis abytes-like objectthat contains initial data.

BytesIOprovides or overrides these methods in addition to those fromBufferedIOBaseandIOBase:

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, theBytesIOobject cannot be resized or closed.

Added in version 3.2.

getvalue()

Returnbytescontaining the entire contents of the buffer.

read1(size=-1,/)

InBytesIO,this is the same asread().

Changed in version 3.7:Thesizeargument is now optional.

readinto1(b,/)

InBytesIO,this is the same asreadinto().

Added in version 3.5.

classio.BufferedReader(raw,buffer_size=DEFAULT_BUFFER_SIZE)

A buffered binary stream providing higher-level access to a readable, non seekableRawIOBaseraw binary stream. It inherits from BufferedIOBase.

When reading data from this object, a larger amount of data may be requested from the underlying raw stream, and kept in an internal buffer. The buffered data can then be returned directly on subsequent reads.

The constructor creates aBufferedReaderfor the given readable rawstream andbuffer_size.Ifbuffer_sizeis omitted, DEFAULT_BUFFER_SIZEis used.

BufferedReaderprovides or overrides these methods in addition to those fromBufferedIOBaseandIOBase:

peek(size=0,/)

Return bytes from the stream without advancing the position. At most one single read on the raw stream is done to satisfy the call. The number of bytes returned may be less or more than requested.

read(size=-1,/)

Read and returnsizebytes, or ifsizeis not given or negative, until EOF or if the read call would block in non-blocking mode.

read1(size=-1,/)

Read and return up tosizebytes 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:Thesizeargument is now optional.

classio.BufferedWriter(raw,buffer_size=DEFAULT_BUFFER_SIZE)

A buffered binary stream providing higher-level access to a writeable, non seekableRawIOBaseraw binary stream. It inherits from BufferedIOBase.

When writing to this object, data is normally placed into an internal buffer. The buffer will be written out to the underlyingRawIOBase object under various conditions, including:

• when the buffer gets too small for all pending data;

• whenflush()is called;

• when aseek()is requested (forBufferedRandomobjects);

• when theBufferedWriterobject is closed or destroyed.

The constructor creates aBufferedWriterfor the given writeable rawstream. If thebuffer_sizeis not given, it defaults to DEFAULT_BUFFER_SIZE.

BufferedWriterprovides or overrides these methods in addition to those fromBufferedIOBaseandIOBase:

flush()

Force bytes held in the buffer into the raw stream. A BlockingIOErrorshould be raised if the raw stream blocks.

write(b,/)

Write thebytes-like object,b,and return the number of bytes written. When in non-blocking mode, a BlockingIOErroris raised if the buffer needs to be written out but the raw stream blocks.

classio.BufferedRandom(raw,buffer_size=DEFAULT_BUFFER_SIZE)

A buffered binary stream providing higher-level access to a seekable RawIOBaseraw binary stream. It inherits fromBufferedReader andBufferedWriter.

The constructor creates a reader and writer for a seekable raw stream, given in the first argument. If thebuffer_sizeis omitted it defaults to DEFAULT_BUFFER_SIZE.

BufferedRandomis capable of anythingBufferedReaderor BufferedWritercan do. In addition,seek()and tell()are guaranteed to be implemented.

classio.BufferedRWPair(reader,writer,buffer_size=DEFAULT_BUFFER_SIZE,/)

A buffered binary stream providing higher-level access to two non seekable RawIOBaseraw binary streams—one readable, the other writeable. It inherits fromBufferedIOBase.

readerandwriterareRawIOBaseobjects that are readable and writeable respectively. If thebuffer_sizeis omitted it defaults to DEFAULT_BUFFER_SIZE.

BufferedRWPairimplements all ofBufferedIOBase's methods except fordetach(),which raises UnsupportedOperation.

Warning

BufferedRWPairdoes not attempt to synchronize accesses to its underlying raw streams. You should not pass it the same object as reader and writer; useBufferedRandominstead.

Text I/O

classio.TextIOBase

Base class for text streams. This class provides a character and line based interface to stream I/O. It inherits fromIOBase.

TextIOBaseprovides or overrides these data attributes and methods in addition to those fromIOBase:

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, orNone,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 (aBufferedIOBaseinstance) that TextIOBasedeals with. This is not part of the TextIOBaseAPI and may not exist in some implementations.

detach()

Separate the underlying binary buffer from theTextIOBaseand return it.

After the underlying buffer has been detached, theTextIOBaseis in an unusable state.

SomeTextIOBaseimplementations, likeStringIO,may not have the concept of an underlying buffer and calling this method will raiseUnsupportedOperation.

Added in version 3.1.

read(size=-1,/)

Read and return at mostsizecharacters from the stream as a single str.Ifsizeis negative orNone,reads until EOF.

readline(size=-1,/)

Read until newline or EOF and return a singlestr.If the stream is already at EOF, an empty string is returned.

Ifsizeis specified, at mostsizecharacters will be read.

seek(offset,whence=SEEK_SET,/)

Change the stream position to the givenoffset.Behaviour depends on thewhenceparameter. The default value forwhenceis SEEK_SET.

• SEEK_SETor0:seek from the start of the stream (the default);offsetmust either be a number returned by TextIOBase.tell(),or zero. Any otheroffsetvalue produces undefined behaviour.

• SEEK_CURor1:“seek” to the current position; offsetmust be zero, which is a no-operation (all other values are unsupported).

• SEEK_ENDor2:seek to the end of the stream; offsetmust be zero (all other values are unsupported).

Return the new absolute position as an opaque number.

Added in version 3.1:TheSEEK_*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 stringsto the stream and return the number of characters written.

classio.TextIOWrapper(buffer,encoding=None,errors=None,newline=None,line_buffering=False,write_through=False)

A buffered text stream providing higher-level access to a BufferedIOBasebuffered binary stream. It inherits from TextIOBase.

encodinggives the name of the encoding that the stream will be decoded or encoded with. It defaults tolocale.getencoding(). encoding= "locale"can be used to specify the current locale’s encoding explicitly. SeeText Encodingfor more information.

errorsis an optional string that specifies how encoding and decoding errors are to be handled. Pass'strict'to raise aValueError exception if there is an encoding error (the default ofNonehas 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.

newlinecontrols how line endings are handled. It can beNone, '','\n','\r',and'\r\n'.It works as follows:

• When reading input from the stream, ifnewlineisNone, universal newlinesmode 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. Ifnewlineis'',universal newlines mode is enabled, but line endings are returned to the caller untranslated. Ifnewlinehas 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, ifnewlineisNone,any'\n' characters written are translated to the system default line separator, os.linesep.Ifnewlineis''or'\n',no translation takes place. Ifnewlineis any of the other legal values, any'\n' characters written are translated to the given string.

Ifline_bufferingisTrue,flush()is implied when a call to write contains a newline character or a carriage return.

Ifwrite_throughisTrue,calls towrite()are guaranteed not to be buffered: any data written on theTextIOWrapper object is immediately handled to its underlying binarybuffer.

Changed in version 3.3:Thewrite_throughargument has been added.

Changed in version 3.3:The defaultencodingis nowlocale.getpreferredencoding(False) instead oflocale.getpreferredencoding().Don’t change temporary the locale encoding usinglocale.setlocale(),use the current locale encoding instead of the user preferred encoding.

Changed in version 3.10:Theencodingargument now supports the"locale"dummy encoding name.

TextIOWrapperprovides these data attributes and methods in addition to those fromTextIOBaseandIOBase:

line_buffering

Whether line buffering is enabled.

write_through

Whether writes are passed immediately to the underlying binary buffer.

Added in version 3.7.

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

Reconfigure this text stream using new settings forencoding, errors,newline,line_bufferingandwrite_through.

Parameters not specified keep current settings, except errors='strict'is used whenencodingis specified but errorsis not specified.

It is not possible to change the encoding or newline if some data has already been read from the stream. On the other hand, changing encoding after write is possible.

This method does an implicit stream flush before setting the new parameters.

Added in version 3.7.

Changed in version 3.11:The method supportsencoding= "locale"option.

seek(cookie,whence=os.SEEK_SET,/)

Set the stream position. Return the new stream position as anint.

Four operations are supported, given by the following argument combinations:

• seek(0,SEEK_SET):Rewind to the start of the stream.

• seek(cookie,SEEK_SET):Restore a previous position; cookiemust bea number returned bytell().

• seek(0,SEEK_END):Fast-forward to the end of the stream.

• seek(0,SEEK_CUR):Leave the current stream position unchanged.

Any other argument combinations are invalid, and may raise exceptions.

See also

os.SEEK_SET,os.SEEK_CUR,andos.SEEK_END.

tell()

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

classio.StringIO(initial_value='',newline='\n')

A text stream using an in-memory text buffer. It inherits from TextIOBase.

The text buffer is discarded when theclose()method is called.

The initial value of the buffer can be set by providinginitial_value. If newline translation is enabled, newlines will be encoded as if by write().The stream is positioned at the start of the buffer which emulates opening an existing file in aw+mode, making it ready for an immediate write from the beginning or for a write that would overwrite the initial value. To emulate opening a file in ana+ mode ready for appending, usef.seek(0,io.SEEK_END)to reposition the stream at the end of the buffer.

Thenewlineargument works like that ofTextIOWrapper, except that when writing output to the stream, ifnewlineisNone, newlines are written as\non all platforms.

StringIOprovides this method in addition to those from TextIOBaseandIOBase:

getvalue()

Return astrcontaining the entire contents of the buffer. Newlines are decoded as if byread(),although the stream position is not changed.

Example usage:

importio

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()

classio.IncrementalNewlineDecoder

A helper codec that decodes newlines foruniversal newlinesmode. It inherits fromcodecs.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, tell()andseek()are both quite slow due to the reconstruction algorithm used.

StringIO,however, is a native in-memory unicode container and will exhibit similar speed toBytesIO.

Multi-threading

FileIOobjects are thread-safe to the extent that the operating system calls (such asread(2)under Unix) they wrap are thread-safe too.

Binary buffered objects (instances ofBufferedReader, BufferedWriter,BufferedRandomandBufferedRWPair) protect their internal structures using a lock; it is therefore safe to call them from multiple threads at once.

TextIOWrapperobjects are not thread-safe.

Reentrancy

Binary buffered objects (instances ofBufferedReader, BufferedWriter,BufferedRandomandBufferedRWPair) are not reentrant. While reentrant calls will not happen in normal situations, they can arise from doing I/O in asignalhandler. If a thread tries to re-enter a buffered object which it is already accessing, aRuntimeError is raised. Note this doesn’t prohibit a different thread from entering the buffered object.

The above implicitly extends to text files, since theopen()function will wrap a buffered object inside aTextIOWrapper.This includes standard streams and therefore affects the built-inprint()function as well.