Source code:
———
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
. 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
object to the write() method of a binary stream will raise a
. So will giving a
object to the write() method of a text stream.
Changed in version 3.3: Operations that used to raise
now raise
, since IOError is now an alias of OSError.
Text I/O
Text I/O expects and produces
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
, optionally specifying an encoding:
f=open("myfile.txt","r",encoding="utf-8")In-memory text streams are also available as
objects:
f=io.StringIO("some initial text data")Note
When working with a non-blocking stream, be aware that read operations on text I/O objects might raise a
if the stream cannot perform the operation immediately.
The text stream API is described in detail in the documentation of
.
Binary I/O
Binary I/O (also called buffered I/O) expects
and produces
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
with 'b' in the mode string:
f=open("myfile.jpg","rb")In-memory binary streams are also available as
objects:
f=io.BytesIO(b"some initial binary data: \x00\x01")The binary stream API is described in detail in the docs of
.
Other library modules may provide additional ways to create text or binary streams. See
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
.
Warning
Raw I/O is a low-level interface and methods generally must have their return values checked and be explicitly retried to ensure an operation completes. For instance
returns the number of bytes written which may be less than the number of bytes provided (a partial write). High-level I/O objects like
and
implement retry behavior.
Text Encoding
The default encoding of
and
is locale-specific (
).
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.
Opt-in EncodingWarning
Added in version 3.10: See
for more details.
To find where the default locale encoding is used, you can enable the
command line option or set the
environment variable, which will emit an
when the default encoding is used.
If you are providing an API that uses
or
and passes encoding=None as a parameter, you can use
so that callers of the API will emit an
if they don’t pass an encoding. 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.
uses the file’s blksize (as obtained by
) 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
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 a
and an absolute path.
The behavior of this function may be overridden by an earlier call to the
. However, assuming that path is a
and an absolute path, open_code(path) should always behave the same as open(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 use
or
and have an encoding=None parameter.
This function returns encoding if it is not None. Otherwise, it returns "locale" or "utf-8" depending on
.
This function emits an
if
sys.flags.warn_default_encoding
is true and encoding is None. stacklevel specifies where the warning is emitted. For example:
defread_text(path,encoding=None):encoding=io.text_encoding(encoding)# stacklevel=2withopen(path,encoding)asf:returnf.read()In this example, an
is emitted for the caller of read_text().
See
for more information.
Added in version 3.10.
Changed in version 3.11: text_encoding() returns “utf-8” when UTF-8 mode is enabled and encoding is None.
exceptionio.BlockingIOError
This is a compatibility alias for the builtin BlockingIOError exception.
exceptionio.UnsupportedOperation
An exception inheriting
and
that is raised when an unsupported operation is called on a stream.
Class hierarchy
The implementation of I/O streams is organized as a hierarchy of classes. First
(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,
provides unoptimized implementations of readinto() and readline().
At the top of the I/O hierarchy is the abstract base class
. 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
if they do not support a given operation.
The
ABC extends
. It deals with the reading and writing of bytes to a stream.
subclasses RawIOBase to provide an interface to files in the machine’s file system.
The
ABC extends
. It deals with buffering on a raw binary stream (
). Its subclasses,
,
, and
buffer raw binary streams that are writable, readable, and both readable and writable, respectively.
provides a buffered interface to seekable streams. Another BufferedIOBase subclass,
, is a stream of in-memory bytes.
The
ABC extends
. It deals with streams whose bytes represent text, and handles encoding and decoding to and from strings.
, which extends TextIOBase, is a buffered text interface to a buffered raw stream (
). Finally,
is an in-memory stream for text.
Argument names are not part of the specification, and only the arguments of
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
fileno, seek, and truncate
close, closed, __enter__, __exit__, flush, isatty, __iter__, __next__, readable, readline, readlines, seekable, tell, writable, and writelines
readinto and write
Inherited
methods, read, and readall
detach, read, read1, and write
Inherited
methods, readinto, and readinto1
detach, read, readline, and write
Inherited
methods, encoding, errors, and newlines
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 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
(or
) when operations they do not support are called.
The basic type used for binary data read from or written to a file is
. Other
are accepted as method arguments too. Text I/O classes work with
data.
Note that calling any method (even inquiries) on a closed stream is undefined. Implementations may raise
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
below.
IOBase is also a context manager and therefore supports the
statement. In this example, file is closed after the with statement’s suite is finished—even if an exception occurs:
withopen('spam.txt','w')asfile: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
.
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
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
.
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
can be used to select the line terminator(s) recognized.
readlines(hint=-1, /)
Read and return a list of lines from the stream. hint can be specified to control the number of lines read: no more lines will be read if the total size (in bytes/characters) of all lines so far exceeds hint.
hint values of 0 or less, as well as None, are treated as no hint.
Note that it’s already possible to iterate on file objects using forlineinfile:... without calling file.readlines().
seek(offset, whence=os.SEEK_SET, /)
Change the stream position to the given byte offset, interpreted relative to the position indicated by whence, and return the new absolute position. Values for whence are:
or 0 – start of the stream (the default); offset should be zero or positive
or 1 – current stream position; offset may be negative
or 2 – end of the stream; offset is usually negative
Added in version 3.1: The SEEK_* constants.
Added in version 3.3: Some operating systems could support additional values, like
or
. 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,
,
and
will raise
.
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
will raise
.
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
method.
classio.RawIOBase
Base class for raw binary streams. It inherits from
.
Raw binary streams typically provide low-level access to an underlying OS device or API, and do not try to encapsulate it in high-level primitives (this functionality is done at a higher-level in buffered binary streams and text streams, described later in this page).
RawIOBase provides these methods in addition to those from
:
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.
Attempts to make only one system call but will retry if interrupted and the signal handler does not raise an exception (see
for the rationale). This means 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
and
.
readall()
Read and return all the bytes from the stream until EOF, using multiple calls to the stream if necessary.
If 0 bytes are returned this indicates end of file. If the object is in non-blocking mode and the underlying
returns None indicating no bytes are available, None is returned.
readinto(b, /)
Read bytes into a pre-allocated, writable
b, and return the number of bytes read. For example, b might be a
.
If 0 is returned and len(b) is not 0, this indicates end of file. If the object is in non-blocking mode and no bytes are available, None is returned.
write(b, /)
Write the given
, 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.
Warning
This function does not ensure all bytes are written or an exception is thrown. Callers may implement that behavior by checking the return value and, if it is less than the length of b, looping with additional write calls until all unwritten bytes are written. High-level I/O objects like
and
implement retry behavior.
classio.BufferedIOBase
Base class for binary streams that support some kind of buffering. It inherits from
.
The main difference with
is that methods
,
and
will try (respectively) to read as much input as requested or to emit all provided data.
In addition, if the underlying raw stream is in non-blocking mode, when the system returns would block
will raise
with
BlockingIOError.characters_written
and
will return data read so far or None if no data is available.
Besides, the
method does not have a default implementation that defers to
.
A typical BufferedIOBase implementation should not inherit from a
implementation, but wrap one, like
and
do.
BufferedIOBase provides or overrides these data attributes and methods in addition to those from
:
raw
The underlying raw stream (a
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
, do not have the concept of a single raw stream to return from this method. They raise
.
Added in version 3.1.
read(size=-1, /)
Read and return up to size bytes. If the argument is omitted, None, or negative read as much as possible.
Fewer bytes may be returned than requested. An empty
object is returned if the stream is already at EOF. More than one read may be made and calls may be retried if specific errors are encountered, see
and
for more details. Less than size bytes being returned does not imply that EOF is imminent.
When reading as much as possible the default implementation will use raw.readall if available (which should implement
), otherwise will read in a loop until read returns None, an empty
, or a non-retryable error. For most streams this is to EOF, but for non-blocking streams more data may become available.
Note
When the underlying raw stream is non-blocking, implementations may either raise
or return None if no data is available. io implementations return None.
read1(size=-1, /)
Read and return up to size bytes, calling
which may retry if
is encountered per
. If size is -1 or not provided, the implementation will choose an arbitrary value for size.
Note
When the underlying raw stream is non-blocking, implementations may either raise
or return None if no data is available. io implementations return None.
readinto(b, /)
Read bytes into a pre-allocated, writable
b and return the number of bytes read. For example, b might be a
.
Like
, multiple reads may be issued to the underlying raw stream, unless the latter is interactive.
A
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
b, using at most one call to the underlying raw stream’s
(or
) method. Return the number of bytes read.
A
is raised if the underlying raw stream is in non blocking-mode, and has no data available at the moment.
Added in version 3.5.
write(b, /)
Write the given
, b, and return the number of bytes written (always equal to the length of b in bytes, since if the write fails an
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
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
classio.FileIO(name, mode='r', closefd=True, opener=None)
A raw binary stream representing an OS-level file containing bytes data. It inherits from
and implements its low-level access design. This means
does not guarantee all bytes are written and
may read less bytes than requested even when more bytes may be present in the underlying file. To get “write all” and “read at least” behavior, use
.
The name can be one of two things:
a character string or
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.
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.
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
as opener results in functionality similar to passing None).
The newly created file is
.
See the
built-in function for examples on using the opener parameter.
Warning
FileIO is a low-level I/O object and members, such as
and
, need to have their return values checked explicitly in a retry loop to implement “write all” and “read at least” behavior. High-level I/O objects
and
implement retry behavior.
Changed in version 3.3: The opener parameter was added. The 'x' mode was added.
Changed in version 3.4: The file is now non-inheritable.
FileIO provides these data attributes in addition to those from
and
:
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
. The buffer is discarded when the
method is called.
The optional argument initial_bytes is a
that contains initial data.
BytesIO provides or overrides these methods in addition to those from
and
:
getbuffer()
Return a readable and writable view over the contents of the buffer without copying them. Also, mutating the view will transparently update the contents of the buffer:
>>> b=io.BytesIO(b"abcdef")>>> view=b.getbuffer()>>> view[2:4]=b"56">>> b.getvalue()b'ab56ef'Note
As long as the view exists, the BytesIO object cannot be resized or closed.
Added in version 3.2.
getvalue()
Return
containing the entire contents of the buffer.
read1(size=-1, /)
In BytesIO, this is the same as
.
Changed in version 3.7: The size argument is now optional.
readinto1(b, /)
In BytesIO, this is the same as
.
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 seekable
raw binary stream. It inherits from
.
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,
is used.
BufferedReader provides or overrides these methods in addition to those from
and
:
peek(size=0, /)
Return bytes from the stream without advancing the position. The number of bytes returned may be less or more than requested. If the underlying raw stream is non-blocking and the operation would block, returns empty bytes.
read(size=-1, /)
In BufferedReader this is the same as
read1(size=-1, /)
In BufferedReader this is the same as
Changed in version 3.7: The size argument is now optional.
classio.BufferedWriter(raw, buffer_size=DEFAULT_BUFFER_SIZE)
A buffered binary stream providing higher-level access to a writeable, non seekable
raw binary stream. It inherits from
.
When writing to this object, data is normally placed into an internal buffer. The buffer will be written out to the underlying
object under various conditions, including:
when the buffer gets too small for all pending data;
when
is called;
when a
is requested (for
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
.
BufferedWriter provides or overrides these methods in addition to those from
and
:
flush()
Force bytes held in the buffer into the raw stream. A
should be raised if the raw stream blocks.
write(b, /)
Write the
, b, and return the number of bytes written. When in non-blocking mode, a
with
BlockingIOError.characters_written
set is 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 implementing
interfaces providing higher-level access to a seekable
raw binary stream.
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
.
BufferedRandom is capable of anything
or
can do. In addition,
and
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
raw binary streams—one readable, the other writeable. It inherits from
.
reader and writer are
objects that are readable and writeable respectively. If the buffer_size is omitted it defaults to
.
BufferedRWPair implements all of
's methods except for
, which raises
.
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
instead.
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 from
.
TextIOBase provides or overrides these data attributes and methods in addition to those from
:
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
or
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
, may not have the concept of an underlying buffer and calling this method will raise
.
Added in version 3.1.
read(size=-1, /)
Read and return at most size characters from the stream as a single
. If size is negative or None, reads until EOF.
readline(size=-1, /)
Read until newline or EOF and return a single
. 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
, or zero. Any other offset value produces undefined behaviour.
SEEK_CUR or 1: “seek” to the current position; offset must be zero, which is a no-operation (all other values are unsupported).
SEEK_END or 2: seek to the end of the stream; offset must be zero (all other values are unsupported).
Return the new absolute position as an opaque number.
Added in version 3.1: The SEEK_* constants.
tell()
Return the current stream position as an opaque number. The number does not usually represent a number of bytes in the underlying binary storage.
write(s, /)
Write the string s to the stream and return the number of characters written.
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
buffered binary stream. It inherits from
.
encoding gives the name of the encoding that the stream will be decoded or encoded with. In
, this defaults to UTF-8. Otherwise, it defaults to
. encoding="locale" can be used to specify the current locale’s encoding explicitly. See
for more information.
errors is an optional string that specifies how encoding and decoding errors are to be handled. Pass 'strict' to raise a
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
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,
mode is enabled. Lines in the input can end in '\n', '\r', or '\r\n', and these are translated into '\n' before being returned to the caller. If newline is '', universal newlines mode is enabled, but line endings are returned to the caller untranslated. If newline has any of the other legal values, input lines are only terminated by the given string, and the line ending is returned to the caller untranslated.
When writing output to the stream, if newline is None, any '\n' characters written are translated to the system default line separator,
. 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,
is implied when a call to write contains a newline character or a carriage return.
If write_through is True, calls to
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
, use the current locale encoding instead of the user preferred encoding.
Changed in version 3.10: The encoding argument now supports the "locale" dummy encoding name.
Note
When the underlying raw stream is non-blocking, a
may be raised if a read operation cannot be completed immediately.
TextIOWrapper provides these data attributes and methods in addition to those from
and
:
line_buffering
Whether line buffering is enabled.
write_through
Whether writes are passed immediately to the underlying binary buffer.
Added in version 3.7.
reconfigure(*, encoding=None, errors=None, newline=None, line_buffering=None, write_through=None)
Reconfigure this text stream using new settings for encoding, errors, newline, line_buffering and write_through.
Parameters not specified keep current settings, except errors='strict' is used when encoding is specified but errors is not specified.
It is not possible to change the encoding or newline if some data has already been read from the stream. On the other hand, changing encoding after write is possible.
This method does an implicit stream flush before setting the new parameters.
Added in version 3.7.
Changed in version 3.11: The method supports encoding="locale" option.
seek(cookie, whence=os.SEEK_SET, /)
Set the stream position. Return the new stream position as an
.
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 be a number returned by
.
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.
tell()
Return the stream position as an opaque number. The return value of tell() can be given as input to
, to restore a previous stream position.
classio.StringIO(initial_value='', newline='\n')
A text stream using an in-memory text buffer. It inherits from
.
The text buffer is discarded when the
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
. The stream is positioned at the start of the buffer which emulates opening an existing file in a w+ mode, making it ready for an immediate write from the beginning or for a write that would overwrite the initial value. To emulate opening a file in an a+ mode ready for appending, use f.seek(0,io.SEEK_END) to reposition the stream at the end of the buffer.
The newline argument works like that of
, except that when writing output to the stream, if newline is None, newlines are written as \n on all platforms.
StringIO provides this method in addition to those from
and
:
getvalue()
Return a
containing the entire contents of the buffer. Newlines are decoded as if by
, although the stream position is not changed.
Example usage:
importiooutput=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 for
mode. It inherits from
.
Static Typing
The following protocols can be used for annotating function and method arguments for simple stream reading or writing operations. They are decorated with
.
classio.Reader[T]
Generic protocol for reading from a file or other input stream. T will usually be
or
, but can be any type that is read from the stream.
Added in version 3.14.
read()
read(size, /)Read data from the input stream and return it. If size is specified, it should be an integer, and at most size items (bytes/characters) will be read.
For example:
defread_it(reader:Reader[str]):data=reader.read(11)assertisinstance(data,str)classio.Writer[T]
Generic protocol for writing to a file or other output stream. T will usually be
or
, but can be any type that can be written to the stream.
Added in version 3.14.
write(data, /)
Write data to the output stream and return the number of items (bytes/characters) written.
For example:
defwrite_binary(writer:Writer[bytes]):writer.write(b"Hello world!\n")See
ABCs and Protocols for working with I/O
for other I/O related protocols and classes that can be used for static type checking.
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,
and
are both quite slow due to the reconstruction algorithm used.
, however, is a native in-memory unicode container and will exhibit similar speed to
.
Multi-threading
objects are thread-safe to the extent that the operating system calls (such as
under Unix) they wrap are thread-safe too.
Binary buffered objects (instances of
,
,
and
) protect their internal structures using a lock; it is therefore safe to call them from multiple threads at once.
objects are not thread-safe.
Reentrancy
Binary buffered objects (instances of
,
,
and
) are not reentrant. While reentrant calls will not happen in normal situations, they can arise from doing I/O in a
handler. If a thread tries to re-enter a buffered object which it is already accessing, a
is raised. Note this doesn’t prohibit a different thread from entering the buffered object.
The above implicitly extends to text files, since the
function will wrap a buffered object inside a
. This includes standard streams and therefore affects the built-in
function as well.