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# 15.2. `io` — Core tools for working with streams

## 15.2.1. Overview

The `io` module provides Python’s main facilities for dealing for various types of I/O. There are three main types of I/O: text I/O, binary I/O, raw I/O. These are generic categories, and various backing stores can be used for each of them. Concrete objects belonging to any of these categories will often be called streams; another common term is file-like objects.

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.

### 15.2.1.1. 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 for the `TextIOBase`.

### 15.2.1.2. 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.

### 15.2.1.3. 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`.

## 15.2.2. 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)

  This is an alias for the builtin `open()` function.

exception `io.``BlockingIOError`

  Error raised when blocking would occur on a non-blocking stream. It inherits `IOError`.

  In addition to those of `IOError`, `BlockingIOError` has one attribute:

  `characters_written`

    An integer containing the number of characters written to the stream before it blocked.

exception `io.``UnsupportedOperation`

  An exception inheriting `IOError` and `ValueError` that is raised when an unsupported operation is called on a stream.

### 15.2.2.1. In-memory streams

It is also possible to use a `str` or `bytes`-like object as a file for both reading and writing. For strings `StringIO` can be used like a file opened in text mode. `BytesIO` can be used like a file opened in binary mode. Both provide full read-write capabilities with random access.

See also

`sys`
  contains the standard IO streams: `sys.stdin`, `sys.stdout`, and `sys.stderr`.

## 15.2.3. 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.

> Note

> The abstract base classes also provide default implementations of some methods in order to help implementation of concrete stream classes. For example, `BufferedIOBase` provides unoptimized implementations of `readinto()` and `readline()`.

At the top of the I/O hierarchy is the abstract base class `IOBase`. It defines the basic interface to a stream. Note, however, that there is no separation between reading and writing to streams; implementations are allowed to raise `UnsupportedOperation` if they do not support a given operation.

The `RawIOBase` ABC extends `IOBase`. It deals with the reading and writing of bytes to a stream. `FileIO` subclasses `RawIOBase` to provide an interface to files in the machine’s file system.

The `BufferedIOBase` ABC deals with buffering on a raw byte stream (`RawIOBase`). Its subclasses, `BufferedWriter`, `BufferedReader`, and `BufferedRWPair` buffer streams that are readable, writable, and both readable and writable. `BufferedRandom` provides a buffered interface to random access streams. Another `BufferedIOBase` subclass, `BytesIO`, is a stream of in-memory bytes.

The `TextIOBase` ABC, another subclass of `IOBase`, deals with streams whose bytes represent text, and handles encoding and decoding to and from strings. `TextIOWrapper`, which extends it, is a buffered text interface to a buffered raw stream (`BufferedIOBase`). Finally, `StringIO` is an in-memory stream for text.

Argument names are not part of the specification, and only the arguments of `open()` are intended to be used as keyword arguments.

### 15.2.3.1. 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 `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`. `bytearray`s 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. Lines are defined slightly differently depending on whether the stream is a binary stream (yielding bytes), or a text stream (yielding character strings). See `readline()` below.

  IOBase is also a context manager and therefore supports the `with` statement. In this example, file is closed after the `with` statement’s suite is finished—even if an exception occurs:

      with open('spam.txt', 'w') as file:
          file.write('Spam and eggs!')

  `IOBase` provides these data attributes and methods:

  `close`()

    Flush and close this stream. This method has no effect if the file is already closed. Once the file is closed, any operation on the file (e.g. reading or writing) will raise a `ValueError`.

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

  `closed`

    True if the stream is closed.

  `fileno`()

    Return the underlying file descriptor (an integer) of the stream if it exists. An `IOError` 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 `IOError`.

  `readline`(limit=-1)

    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.

  `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.

  `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.

  `seekable`()

    Return `True` if the stream supports random access. If `False`, `seek()`, `tell()` and `truncate()` will raise `IOError`.

  `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, on Windows they’re undetermined). The new file size is returned.

  `writable`()

    Return `True` if the stream supports writing. If `False`, `write()` and `truncate()` will raise `IOError`.

  `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.

class `io.``RawIOBase`

  Base class for raw binary I/O. It inherits `IOBase`. There is no public constructor.

  Raw binary I/O typically provides low-level access to an underlying OS device or API, and does not try to encapsulate it in high-level primitives (this is left to Buffered I/O and Text I/O, described later in this page).

  In addition to the attributes and methods from `IOBase`, RawIOBase provides the following methods:

  `read`(n=-1)

    Read up to n bytes from the object and return them. As a convenience, if n is unspecified or -1, `readall()` is called. Otherwise, only one system call is ever made. Fewer than n bytes may be returned if the operating system call returns fewer than n bytes.

    If 0 bytes are returned, and n was not 0, this indicates end of file. If the object is in non-blocking mode and no bytes are available, `None` is returned.

  `readall`()

    Read and return all the bytes from the stream until EOF, using multiple calls to the stream if necessary.

  `readinto`(b)

    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`(b)

    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`.

  Besides, the `read()` method does not have a default implementation that defers to `readinto()`.

  A typical `BufferedIOBase` implementation should not inherit from a `RawIOBase` implementation, but wrap one, like `BufferedWriter` and `BufferedReader` do.

  `BufferedIOBase` provides or overrides these members in addition to those from `IOBase`:

  `raw`

    The underlying raw stream (a `RawIOBase` instance) that `BufferedIOBase` deals with. This is not part of the `BufferedIOBase` API and may not exist on some implementations.

  `detach`()

    Separate the underlying raw stream from the buffer and return it.

    After the raw stream has been detached, the buffer is in an unusable state.

    Some buffers, like `BytesIO`, do not have the concept of a single raw stream to return from this method. They raise `UnsupportedOperation`.

    New in version 3.1.

  `read`(n=-1)

    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 is in non blocking-mode, and has no data available at the moment.

  `read1`(n=-1)

    Read and return up to n bytes, with at most one call to the underlying raw stream’s `read()` method. This can be useful if you are implementing your own buffering on top of a `BufferedIOBase` object.

  `readinto`(b)

    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`(b)

    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 `IOError` 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.

### 15.2.3.2. Raw File I/O

class `io.``FileIO`(name, mode='r', closefd=True)

  `FileIO` represents an OS-level file containing bytes data. It implements the `RawIOBase` interface (and therefore the `IOBase` interface, too).

  The name can be one of two things:

  - a character string or bytes object representing the path to the file which will be opened;
  - 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'` 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.

  The `read()` (when called with a positive argument), `readinto()` and `write()` methods on this class will only make one system call.

  In addition to the attributes and methods from `IOBase` and `RawIOBase`, `FileIO` provides the following data attributes and methods:

  `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.

### 15.2.3.3. 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.

  `BytesIO` provides or overrides these methods in addition to those from `BufferedIOBase` and `IOBase`:

  `getbuffer`()

    Return a readable and writable view over the contents of the buffer without copying them. Also, mutating the view will transparently update the contents of the buffer:

        >>> b = io.BytesIO(b"abcdef")
        >>> view = b.getbuffer()
        >>> view[2:4] = b"56"
        >>> b.getvalue()
        b'ab56ef'

    Note

    As long as the view exists, the `BytesIO` object cannot be resized.

    New in version 3.2.

  `getvalue`()

    Return `bytes` containing the entire contents of the buffer.

  `read1`()

    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.

  The constructor creates a `BufferedReader` for the given readable raw stream and buffer_size. If buffer_size is omitted, `DEFAULT_BUFFER_SIZE` is used.

  `BufferedReader` provides or overrides these methods in addition to those from `BufferedIOBase` and `IOBase`:

  `peek`([n])

    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`([n])

    Read and return n bytes, or if n is not given or negative, until EOF or if the read call would block in non-blocking mode.

  `read1`(n)

    Read and return up to n 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 held into an internal buffer. The buffer will be written out to the underlying `RawIOBase` object under various conditions, including:

  - when the buffer gets too small for all pending data;
  - when `flush()` is called;
  - when a `seek()` is requested (for `BufferedRandom` objects);
  - when the `BufferedWriter` object is closed or destroyed.

  The constructor creates a `BufferedWriter` for the given writeable raw stream. If the buffer_size is not given, it defaults to `DEFAULT_BUFFER_SIZE`.

  A third argument, max_buffer_size, is supported, but unused and deprecated.

  `BufferedWriter` provides or overrides these methods in addition to those from `BufferedIOBase` and `IOBase`:

  `flush`()

    Force bytes held in the buffer into the raw stream. A `BlockingIOError` should be raised if the raw stream blocks.

  `write`(b)

    Write the bytes or bytearray object, b and return the number of bytes written. When in non-blocking mode, a `BlockingIOError` is raised if the buffer needs to be written out but the raw stream blocks.

class `io.``BufferedRWPair`(reader, writer, buffer_size=DEFAULT_BUFFER_SIZE)

  A buffered I/O object giving a combined, higher-level access to two sequential `RawIOBase` objects: one readable, the other writeable. It is useful for pairs of unidirectional communication channels (pipes, for instance). 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`.

  A fourth argument, max_buffer_size, is supported, but unused and deprecated.

  `BufferedRWPair` implements all of `BufferedIOBase`‘s methods except for `detach()`, which raises `UnsupportedOperation`.

class `io.``BufferedRandom`(raw, buffer_size=DEFAULT_BUFFER_SIZE)

  A buffered interface to random access streams. It inherits `BufferedReader` and `BufferedWriter`, and further supports `seek()` and `tell()` functionality.

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

  A third argument, max_buffer_size, is supported, but unused and deprecated.

  `BufferedRandom` is capable of anything `BufferedReader` or `BufferedWriter` can do.

### 15.2.3.4. Text I/O

class `io.``TextIOBase`

  Base class for text streams. This class provides a character and line based interface to stream I/O. There is no `readinto()` method because Python’s character strings are immutable. It inherits `IOBase`. There is no public constructor.

  `TextIOBase` provides or overrides these data attributes and methods in addition to those from `IOBase`:

  `encoding`

    The name of the encoding used to decode the stream’s bytes into strings, and to encode strings into bytes.

  `errors`

    The error setting of the decoder or encoder.

  `newlines`

    A string, a tuple of strings, or `None`, indicating the newlines translated so far. Depending on the implementation and the initial constructor flags, this may not be available.

  `buffer`

    The underlying binary buffer (a `BufferedIOBase` instance) that `TextIOBase` deals with. This is not part of the `TextIOBase` API and may not exist on some implementations.

  `detach`()

    Separate the underlying binary buffer from the `TextIOBase` and return it.

    After the underlying buffer has been detached, the `TextIOBase` is in an unusable state.

    Some `TextIOBase` implementations, like `StringIO`, may not have the concept of an underlying buffer and calling this method will raise `UnsupportedOperation`.

    New in version 3.1.

  `read`(n)

    Read and return at most n characters from the stream as a single `str`. If n is negative or `None`, reads until EOF.

  `readline`()

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

  `write`(s)

    Write the string s to the stream and return the number of characters written.

class `io.``TextIOWrapper`(buffer, encoding=None, errors=None, newline=None, line_buffering=False)

  A buffered text stream over a `BufferedIOBase` binary stream. It inherits `TextIOBase`.

  encoding gives the name of the encoding that the stream will be decoded or encoded with. It defaults to `locale.getpreferredencoding()`.

  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 separator, `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 one attribute in addition to those of `TextIOBase` and its parents:

  `line_buffering`

    Whether line buffering is enabled.

class `io.``StringIO`(initial_value='', newline=None)

  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 do no newline translation.

  `StringIO` provides this method in addition to those from `TextIOBase` and its parents:

  `getvalue`()

    Return a `str` containing the entire contents of the buffer at any time before the `StringIO` object’s `close()` method is called.

  Example usage:

      import io

      output = io.StringIO()
      output.write('First line.\n')
      print('Second line.', file=output)

      # Retrieve file contents -- this will be
      # 'First line.\nSecond line.\n'
      contents = output.getvalue()

      # Close object and discard memory buffer --
      # .getvalue() will now raise an exception.
      output.close()

class `io.``IncrementalNewlineDecoder`

  A helper codec that decodes newlines for universal newlines mode. It inherits `codecs.IncrementalDecoder`.

## 15.2.4. Advanced topics

Here we will discuss several advanced topics pertaining to the concrete I/O implementations described above.

### 15.2.4.1. Performance

#### 15.2.4.1.1. Binary I/O

By reading and writing only large chunks of data even when the user asks for a single byte, buffered I/O is designed to hide any inefficiency in calling and executing the operating system’s unbuffered I/O routines. The gain will vary very much depending on the OS and the kind of I/O which is performed (for example, on some contemporary 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 will offer you predictable performance regardless of the platform and the backing device. Therefore, it is most always preferable to use buffered I/O rather than unbuffered I/O.

#### 15.2.4.1.2. 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 implies conversions from unicode to binary data using a character codec. This can become noticeable if you handle huge amounts of text data (for example very large log files). Also, `TextIOWrapper.tell()` and `TextIOWrapper.seek()` are both quite slow due to the reconstruction algorithm used.

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

### 15.2.4.2. Multi-threading

`FileIO` objects are thread-safe to the extent that the operating system calls (such as `read(2)` under Unix) they are wrapping 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.

### 15.2.4.3. Reentrancy

Binary buffered objects (instances of `BufferedReader`, `BufferedWriter`, `BufferedRandom` and `BufferedRWPair`) are not reentrant. While reentrant calls will not happen in normal situations, they can arise if you are doing I/O in a `signal` handler. If it is attempted to enter a buffered object again while already being accessed from the same thread, then a `RuntimeError` is raised.

The above implicitly extends to text files, since the `open()` function will wrap a buffered object inside a `TextIOWrapper`. This includes standard streams and therefore affects the built-in function `print()` as well.

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