16.2. io — Core tools for working with streams
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.
18.104.22.168. 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.
22.214.171.124. Binary I/O
Binary I/O (also called buffered I/O) expects and produces bytes objects. No encoding, decoding, or newline translation is performed. This category of streams can be used for all kinds of non-text data, and also when manual control over the handling of text data is desired.
The easiest way to create a binary stream is with open() with 'b' in the mode string:
f = open("myfile.jpg", "rb")
In-memory binary streams are also available as BytesIO objects:
f = io.BytesIO(b"some initial binary data: \x00\x01")
The binary stream API is described in detail in the docs of BufferedIOBase.
Other library modules may provide additional ways to create text or binary streams. See socket.socket.makefile() for example.
126.96.36.199. Raw I/O
Raw I/O (also called unbuffered I/O) is generally used as a low-level building-block for binary and text streams; it is rarely useful to directly manipulate a raw stream from user code. Nevertheless, you can create a raw stream by opening a file in binary mode with buffering disabled:
f = open("myfile.jpg", "rb", buffering=0)
The raw stream API is described in detail in the docs of RawIOBase.
16.2.2. High-level Module Interface
- io.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
188.8.131.52. 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.
16.2.3. 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.
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 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 truncate||close, closed, __enter__, __exit__, 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, read1, and write||Inherited IOBase methods, readinto|
|TextIOBase||IOBase||detach, read, readline, and write||Inherited IOBase methods, encoding, errors, and newlines|
184.108.40.206. 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.
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.
with open('spam.txt', 'w') as file: file.write('Spam and eggs!')
IOBase provides these data attributes and methods:
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.
True if the stream is closed.
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 the write buffers of the stream if applicable. This does nothing for read-only and non-blocking streams.
Return True if the stream is interactive (i.e., connected to a terminal/tty device).
Return True if the stream can be read from. If False, read() will raise OSError.
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 newlines argument to open() can be used to select the line terminator(s) recognized.
Read and return a list of lines from the stream. hint can be specified to control the number of lines read: no more lines will be read if the total size (in bytes/characters) of all lines so far exceeds hint.
Note that it’s already possible to iterate on file objects using for line in file: ... without calling file.readlines().
- seek(offset, whence=SEEK_SET)
Change the stream position to the given byte offset. offset is interpreted relative to the position indicated by whence. 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.
Return the current stream position.
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, on Windows they’re undetermined). The new file size is returned.
Write a list of lines to the stream. Line separators are not added, so it is usual for each of the lines provided to have a line separator at the end.
- class io.RawIOBase
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).
Read up to size bytes from the object and return them. As a convenience, if size is unspecified or -1, readall() is called. Otherwise, only one system call is ever made. Fewer than size bytes may be returned if the operating system call returns fewer than size bytes.
If 0 bytes are returned, and size was not 0, this indicates end of file. If the object is in non-blocking mode and no bytes are available, None is returned.
Read and return all the bytes from the stream until EOF, using multiple calls to the stream if necessary.
Read up to len(b) bytes into bytearray b and return the number of bytes read. If the object is in non-blocking mode and no bytes are available, None is returned.
Write the given bytes or bytearray object, b, to the underlying raw stream and return the number of bytes written. This can be less than len(b), 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.
- 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.
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.
New in version 3.1.
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.
Read up to len(b) bytes into bytearray b and return the number of bytes read.
Like read(), multiple reads may be issued to the underlying raw stream, unless the latter is interactive.
A BlockingIOError is raised if the underlying raw stream is in non blocking-mode, and has no data available at the moment.
Write the given bytes or bytearray object, b and return the number of bytes written (never less than len(b), since if the write fails an OSError will be raised). Depending on the actual implementation, these bytes may be readily written to the underlying stream, or held in a buffer for performance and latency reasons.
When in non-blocking mode, a BlockingIOError is raised if the data needed to be written to the raw stream but it couldn’t accept all the data without blocking.
220.127.116.11. Raw File I/O
- class io.FileIO(name, mode='r', closefd=True, opener=None)
The name can be one of two things:
- a character string or bytes object representing the path to the file which will be opened;
- an integer representing the number of an existing OS-level file descriptor to which the resulting FileIO object will give access.
The mode can be 'r', 'w', 'x' or 'a' for reading (default), writing, exclusive creation or appending. The file will be created if it doesn’t exist when opened for writing or appending; it will be truncated when opened for writing. FileExistsError will be raised if it already exists when opened for creating. Opening a file for creating implies writing, so this mode behaves in a similar way to 'w'. Add a '+' to the mode to allow simultaneous reading and writing.
The read() (when called with a positive argument), readinto() and write() methods on this class will only make one system call.
A custom opener can be used by passing a callable as opener. The underlying file descriptor for the file object is then obtained by calling opener with (name, flags). opener must return an open file descriptor (passing os.open as opener results in functionality similar to passing None).
The newly created file is non-inheritable.
See the open() built-in function for examples on using the opener parameter.
Changed in version 3.3: The opener parameter was added. The 'x' mode was added.
Changed in version 3.4: The file is now non-inheritable.
The mode as given in the constructor.
The file name. This is the file descriptor of the file when no name is given in the constructor.
18.104.22.168. 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 argument initial_bytes contains optional initial bytes data.
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'
As long as the view exists, the BytesIO object cannot be resized.
New in version 3.2.
Return bytes containing the entire contents of the buffer.
In BytesIO, this is the same as read().
- 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.
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 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.
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.
Force bytes held in the buffer into the raw stream. A BlockingIOError should be raised if the raw stream blocks.
- class io.BufferedRandom(raw, buffer_size=DEFAULT_BUFFER_SIZE)
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.
- class io.BufferedRWPair(reader, writer, buffer_size=DEFAULT_BUFFER_SIZE)
22.214.171.124. 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.
The name of the encoding used to decode the stream’s bytes into strings, and to encode strings into bytes.
The error setting of the decoder or encoder.
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.
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.
New in version 3.1.
Read and return at most size characters from the stream as a single str. If size is negative or None, reads until EOF.
Read until newline or EOF and return a single str. If the stream is already at EOF, an empty string is returned.
If size is specified, at most size characters will be read.
- seek(offset, whence=SEEK_SET)
Change the stream position to the given offset. Behaviour depends on the whence parameter:
- 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.
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 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)
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. When writing, 'xmlcharrefreplace' (replace with the appropriate XML character reference) or 'backslashreplace' (replace with backslashed 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.
Whether line buffering is enabled.
- class io.StringIO(initial_value='', newline='\n')
An in-memory stream for text I/O.
The initial value of the buffer (an empty string by default) can be set by providing initial_value. The newline argument works like that of TextIOWrapper. The default is to consider only \n characters as end of lines and to do no newline translation.
Return a str containing the entire contents of the buffer at any time before the StringIO object’s close() method is called.
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()
This section discusses the performance of the provided concrete I/O implementations.
126.96.36.199. 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.
188.8.131.52. 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.
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.
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.