The io module provides the Python interfaces to stream handling. The builtin open() function is defined in this module.
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 seperation between reading and writing to streams; implementations are allowed to throw an IOError if they do not support a given operation.
Extending IOBase is RawIOBase which deals simply with the reading and writing of raw bytes to a stream. FileIO subclasses RawIOBase to provide an interface to files in the machine’s file system.
BufferedIOBase 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. BytesIO is a simple stream of in-memory bytes.
Another IOBase subclass, TextIOBase, deals with streams whose bytes represent text, and handles encoding and decoding from and to 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.
Open file and return a stream. If the file cannot be opened, an IOError is raised.
file is either a string giving the name (and the path if the file isn’t in the current working directory) of the file to be opened or a file descriptor of the file to be opened. (If a file descriptor is given, for example, from os.fdopen(), it is closed when the returned I/O object is closed, unless closefd is set to False.)
mode is an optional string that specifies the mode in which the file is opened. It defaults to 'r' which means open for reading in text mode. Other common values are 'w' for writing (truncating the file if it already exists), and 'a' for appending (which on some Unix systems, means that all writes append to the end of the file regardless of the current seek position). In text mode, if encoding is not specified the encoding used is platform dependent. (For reading and writing raw bytes use binary mode and leave encoding unspecified.) The available modes are:
| Character | Meaning |
| 'r' | open for reading (default) |
| 'w' | open for writing, truncating the file first |
| 'a' | open for writing, appending to the end of the file if it exists |
| 'b' | binary mode |
| 't' | text mode (default) |
| ' ' | open a disk file for updating (reading and writing) |
| 'U' | universal newline mode (for backwards compatibility; should not be used in new code) |
The default mode is 'rt' (open for reading text). For binary random access, the mode 'w b' opens and truncates the file to 0 bytes, while 'r b' opens the file without truncation.
Python distinguishes between files opened in binary and text modes, even when the underlying operating system doesn’t. Files opened in binary mode (including 'b' in the mode argument) return contents as bytes objects without any decoding. In text mode (the default, or when 't' is included in the mode argument), the contents of the file are returned as strings, the bytes having been first decoded using a platform-dependent encoding or using the specified encoding if given.
buffering is an optional integer used to set the buffering policy. By default full buffering is on. Pass 0 to switch buffering off (only allowed in binary mode), 1 to set line buffering, and an integer > 1 for full buffering.
encoding is the name of the encoding used to decode or encode the file. This should only be used in text mode. The default encoding is platform dependent, but any encoding supported by Python can be used. See the codecs module for the list of supported encodings.
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 universal newlines works (it only applies to text mode). It can be None, '', '\n', '\r', and '\r\n'. It works as follows:
If closefd is False and a file descriptor rather than a filename was given, the underlying file descriptor will be kept open when the file is closed. If a filename is given closefd has no effect but must be True (the default).
The type of file object returned by the open() function depends on the mode. When open() is used to open a file in a text mode ('w', 'r', 'wt', 'rt', etc.), it returns a TextIOWrapper. When used to open a file in a binary mode, the returned class varies: in read binary mode, it returns a BufferedReader; in write binary and append binary modes, it returns a BufferedWriter, and in read/write mode, it returns a BufferedRandom.
It is also possible to use a string or bytearray as a file for both reading and writing. For strings StringIO can be used like a file opened in a text mode, and for bytearrays a BytesIO can be used like a file opened in a binary mode.
Error raised when blocking would occur on a non-blocking stream. It inherits IOError.
In addition to those of IOError, BlockingIOError has one attribute:
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 IOError when operations they do not support are called.
The basic type used for binary data read from or written to a file is bytes. bytearrays are accepted too, and in some cases (such as readinto) required. Text I/O classes work with str data.
Note that calling any method (even inquiries) on a closed stream is undefined. Implementations may raise IOError in this case.
IOBase (and its subclasses) support the iterator protocol, meaning that an IOBase object can be iterated over yielding the lines in a stream.
IOBase is also a context manager and therefore supports the with statement. In this example, file is closed after the with statement’s suite is finished—even if an exception occurs:
with open('spam.txt', 'w') as file:
file.write('Spam and eggs!')
IOBase provides these data attributes and methods:
Read and return one line from the stream. If limit is specified, at most limit 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.
Change the stream position to the given byte offset. offset is interpreted relative to the position indicated by whence. Values for whence are:
Return the new absolute position.
Base class for raw binary I/O. It inherits IOBase. There is no public constructor.
In addition to the attributes and methods from IOBase, RawIOBase provides the following methods:
FileIO represents a file containing bytes data. It implements the RawIOBase interface (and therefore the IOBase interface, too).
The mode can be 'r', 'w' or 'a' for reading (default), writing, 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. Add a ' ' to the mode to allow simultaneous reading and writing.
In addition to the attributes and methods from IOBase and RawIOBase, FileIO provides the following data attributes and methods:
Note that the inherited readinto() method should not be used on FileIO objects.
Base class for streams that support buffering. It inherits IOBase. There is no public constructor.
The main difference with RawIOBase is that the read() method supports omitting the size argument, and does not have a default implementation that defers to readinto().
In addition, read(), readinto(), and write() may raise BlockingIOError if the underlying raw stream is in non-blocking mode and not ready; unlike their raw counterparts, they will never return None.
A typical implementation should not inherit from a RawIOBase implementation, but wrap one like BufferedWriter and BufferedReader.
BufferedIOBase provides or overrides these methods in addition to those from IOBase:
Read and return up to n 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 has no data 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 has no data at the moment.
Write the given bytes or bytearray object, b, to the underlying raw stream and return the number of bytes written (never less than len(b), since if the write fails an IOError will be raised).
A BlockingIOError is raised if the buffer is full, and the underlying raw stream cannot accept more data at the moment.
A stream implementation using an in-memory bytes buffer. It inherits BufferedIOBase.
The argument initial_bytes is an optional initial bytearray.
BytesIO provides or overrides these methods in addition to those from BufferedIOBase and IOBase:
A buffer for a readable, sequential RawIOBase object. It inherits BufferedIOBase.
The constructor creates a BufferedReader for the given readable raw stream and buffer_size. If buffer_size is omitted, DEFAULT_BUFFER_SIZE is used.
BufferedReader provides or overrides these methods in addition to those from BufferedIOBase and IOBase:
A buffer for a writeable sequential RawIO object. It inherits BufferedIOBase.
The constructor creates a BufferedWriter for the given writeable raw stream. If the buffer_size is not given, it defaults to DEAFULT_BUFFER_SIZE. If max_buffer_size is omitted, it defaults to twice the buffer size.
BufferedWriter provides or overrides these methods in addition to those from BufferedIOBase and IOBase:
A combined buffered writer and reader object for a raw stream that can be written to and read from. It has and supports both read(), write(), and their variants. This is useful for sockets and two-way pipes. It inherits BufferedIOBase.
reader and writer are RawIOBase objects that are readable and writeable respectively. If the buffer_size is omitted it defaults to DEFAULT_BUFFER_SIZE. The max_buffer_size (for the buffered writer) defaults to twice the buffer size.
BufferedRWPair implements all of BufferedIOBase‘s methods.
A buffered interface to random access streams. It inherits BufferedReader and BufferedWriter.
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. The max_buffer_size (for the buffered writer) defaults to twice the buffer size.
BufferedRandom is capable of anything BufferedReader or BufferedWriter can do.
Base class for text streams. This class provides a character and line based interface to stream I/O. There is no readinto() method because Python’s character strings are immutable. It inherits IOBase. There is no public constructor.
TextIOBase provides or overrides these data attributes and methods in addition to those from IOBase:
A buffered text stream over a BufferedIOBase raw stream, buffer. It inherits TextIOBase.
encoding gives the name of the encoding that the stream will be decoded or encoded with. It defaults to locale.getpreferredencoding().
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 can be None, '', '\n', '\r', or '\r\n'. It controls the handling of line endings. If it is None, universal newlines is enabled. With this enabled, on input, the lines endings '\n', '\r', or '\r\n' are translated to '\n' before being returned to the caller. Conversely, on output, '\n' is translated to the system default line seperator, os.linesep. If newline is any other of its legal values, that newline becomes the newline when the file is read and it is returned untranslated. On output, '\n' is converted to the newline.
If line_buffering is True, flush() is implied when a call to write contains a newline character.
TextIOWrapper provides these data attributes in addition to those of TextIOBase and its parents:
An in-memory stream for text. It inherits TextIOWrapper.
Create a new StringIO stream with an initial value, encoding, error handling, and newline setting. See TextIOWrapper‘s constructor for more information.
StringIO provides this method in addition to those from TextIOWrapper and its parents:
Example usage:
import io
output = io.StringIO()
output.write('First line.\n')
print('Second line.', file=output)
# Retrieve file contents -- this will be
# 'First line.\nSecond line.\n'
contents = output.getvalue()
# Close object and discard memory buffer --
# .getvalue() will now raise an exception.
output.close()