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"""

Python implementation of the io module.

"""

import os

import abc

import codecs

import errno

import stat

import sys

# Import _thread instead of threading to reduce startup cost

from _thread import allocate_lock as Lock

if sys.platform in {'win32', 'cygwin'}:

from msvcrt import setmode as _setmode

else:

_setmode = None

import io

from io import (__all__, SEEK_SET, SEEK_CUR, SEEK_END, Reader, Writer) # noqa: F401

valid_seek_flags = {0, 1, 2} # Hardwired values

if hasattr(os, 'SEEK_HOLE') :

valid_seek_flags.add(os.SEEK_HOLE)

valid_seek_flags.add(os.SEEK_DATA)

# open() uses max(min(blocksize, 8 MiB), DEFAULT_BUFFER_SIZE)

# when the device block size is available.

DEFAULT_BUFFER_SIZE = 128 * 1024 # bytes

# NOTE: Base classes defined here are registered with the "official" ABCs

# defined in io.py. We don't use real inheritance though, because we don't want

# to inherit the C implementations.

# Rebind for compatibility

BlockingIOError = BlockingIOError

# Does open() check its 'errors' argument?

_CHECK_ERRORS = (hasattr(sys, "gettotalrefcount") or sys.flags.dev_mode)

def text_encoding(encoding, stacklevel=2):

"""

A helper function to choose the text encoding.

When encoding is not None, this function returns it.

Otherwise, this function returns the default text encoding

(i.e. "locale" or "utf-8" depends on UTF-8 mode).

This function emits an EncodingWarning if *encoding* is None and

sys.flags.warn_default_encoding is true.

This can be used in APIs with an encoding=None parameter

that pass it to TextIOWrapper or open.

However, please consider using encoding="utf-8" for new APIs.

"""

if encoding is None:

if sys.flags.utf8_mode:

encoding = "utf-8"

else:

encoding = "locale"

if sys.flags.warn_default_encoding:

import warnings

warnings.warn("'encoding' argument not specified.",

EncodingWarning, stacklevel + 1)

return encoding

# Wrapper for builtins.open

# Trick so that open() won't become a bound method when stored

# as a class variable (as dbm.dumb does).

# See init_set_builtins_open() in Python/pylifecycle.c.

@staticmethod

def open(file, mode="r", buffering=-1, encoding=None, errors=None,

newline=None, closefd=True, opener=None):

r"""Open file and return a stream. Raise OSError upon failure.

file is either a text or byte string giving the name (and the path

if the file isn't in the current working directory) of the file to

be opened or an integer file descriptor of the file to be

wrapped. (If a file descriptor is given, 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), 'x' for exclusive creation of a new file, 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

'x' create a new file and open it for writing

'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)

========= ==========================================================

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. The 'x' mode implies 'w' and

raises an `FileExistsError` if the file already exists.

Python distinguishes between files opened in binary and text modes,

even when the underlying operating system doesn't. Files opened in

binary mode (appending 'b' to the mode argument) return contents as

bytes objects without any decoding. In text mode (the default, or when

't' is appended to 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.

Pass 0 to switch buffering off (only allowed in binary mode), 1 to

select line buffering (only usable in text mode), and an integer > 1 to

indicate the size of a fixed-size chunk buffer. When no buffering

argument is given, the default buffering policy works as follows:

* Binary files are buffered in fixed-size chunks; the size of the buffer

is max(min(blocksize, 8 MiB), DEFAULT_BUFFER_SIZE) when the device

block size is available.

On most systems, the buffer will typically be 128 kilobytes long.

* "Interactive" text files (files for which isatty() returns True)

use line buffering. Other text files use the policy described above

for binary files.

encoding is the str 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

passed. See the codecs module for the list of supported encodings.

errors is an optional string that specifies how encoding errors are to

be handled---this argument should not be used in binary mode. 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.)

See the documentation for codecs.register for a list of the permitted

encoding error strings.

newline is a string controlling how universal newlines works (it only

applies to text mode). It can be None, '', '\n', '\r', and '\r\n'. It

works as follows:

* On input, 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 newline 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.

* On output, if newline is None, any '\n' characters written are

translated to the system default line separator, os.linesep. If

newline is '', no translation takes place. If newline is any of the

other legal values, any '\n' characters written are translated to

the given string.

closedfd is a bool. If closefd is False, the underlying file descriptor

will be kept open when the file is closed. This does not work when

a file name is given and must be True in that case.

The newly created file is non-inheritable.

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 (*file*, *flags*). *opener* must return an open

file descriptor (passing os.open as *opener* results in functionality

similar to passing None).

open() returns a file object whose type depends on the mode, and

through which the standard file operations such as reading and writing

are performed. 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 bytes a BytesIO can be used like a file

opened in a binary mode.

"""

if not isinstance(file, int):

file = os.fspath(file)

if not isinstance(file, (str, bytes, int)):

raise TypeError("invalid file: %r" % file)

if not isinstance(mode, str):

raise TypeError("invalid mode: %r" % mode)

if not isinstance(buffering, int):

raise TypeError("invalid buffering: %r" % buffering)

if encoding is not None and not isinstance(encoding, str):

raise TypeError("invalid encoding: %r" % encoding)

if errors is not None and not isinstance(errors, str):

raise TypeError("invalid errors: %r" % errors)

modes = set(mode)

if modes - set("axrwb+t") or len(mode) > len(modes):

raise ValueError("invalid mode: %r" % mode)

creating = "x" in modes

reading = "r" in modes

writing = "w" in modes

appending = "a" in modes

updating = "+" in modes

text = "t" in modes

binary = "b" in modes

if text and binary:

raise ValueError("can't have text and binary mode at once")

if creating + reading + writing + appending > 1:

raise ValueError("can't have read/write/append mode at once")

if not (creating or reading or writing or appending):

raise ValueError("must have exactly one of read/write/append mode")

if binary and encoding is not None:

raise ValueError("binary mode doesn't take an encoding argument")

if binary and errors is not None:

raise ValueError("binary mode doesn't take an errors argument")

if binary and newline is not None:

raise ValueError("binary mode doesn't take a newline argument")

if binary and buffering == 1:

import warnings

warnings.warn("line buffering (buffering=1) isn't supported in binary "

"mode, the default buffer size will be used",

RuntimeWarning, 2)

raw = FileIO(file,

(creating and "x" or "") +

(reading and "r" or "") +

(writing and "w" or "") +

(appending and "a" or "") +

(updating and "+" or ""),

closefd, opener=opener)

result = raw

try:

line_buffering = False

if buffering == 1 or buffering < 0 and raw._isatty_open_only():

buffering = -1

line_buffering = True

if buffering < 0:

buffering = max(min(raw._blksize, 8192 * 1024), DEFAULT_BUFFER_SIZE)

if buffering < 0:

raise ValueError("invalid buffering size")

if buffering == 0:

if binary:

return result

raise ValueError("can't have unbuffered text I/O")

if updating:

buffer = BufferedRandom(raw, buffering)

elif creating or writing or appending:

buffer = BufferedWriter(raw, buffering)

elif reading:

buffer = BufferedReader(raw, buffering)

else:

raise ValueError("unknown mode: %r" % mode)

result = buffer

if binary:

return result

encoding = text_encoding(encoding)

text = TextIOWrapper(buffer, encoding, errors, newline, line_buffering)

result = text

text.mode = mode

return result

except:

result.close()

raise

# Define a default pure-Python implementation for open_code()

# that does not allow hooks. Warn on first use. Defined for tests.

def _open_code_with_warning(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 an absolute path.

When supported by the runtime, this function can be hooked

in order to allow embedders more control over code files.

This functionality is not supported on the current runtime.

"""

import warnings

warnings.warn("_pyio.open_code() may not be using hooks",

RuntimeWarning, 2)

return open(path, "rb")

try:

open_code = io.open_code

except AttributeError:

open_code = _open_code_with_warning

# In normal operation, both `UnsupportedOperation`s should be bound to the

# same object.

try:

UnsupportedOperation = io.UnsupportedOperation

except AttributeError:

class UnsupportedOperation(OSError, ValueError):

pass

class IOBase(metaclass=abc.ABCMeta):

"""The abstract base class for all I/O classes.

This class provides dummy 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 UnsupportedOperation when operations they do not support are

called.

The basic type used for binary data read from or written to a file is

bytes. Other bytes-like objects are accepted as method arguments too.

Text I/O classes work with str data.

Note that calling any method (even inquiries) on a closed stream is

undefined. Implementations may raise OSError 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 also supports the :keyword:`with` statement. In this example,

fp is closed after the suite of the with statement is complete:

with open('spam.txt', 'r') as fp:

fp.write('Spam and eggs!')

"""

### Internal ###

def _unsupported(self, name):

"""Internal: raise an OSError exception for unsupported operations."""

raise UnsupportedOperation("%s.%s() not supported" %

(self.__class__.__name__, name))

### Positioning ###

def seek(self, pos, whence=0):

"""Change stream position.

Change the stream position to byte offset pos. Argument pos is

interpreted relative to the position indicated by whence. Values

for whence are ints:

* 0 -- start of stream (the default); offset should be zero or

positive

* 1 -- current stream position; offset may be negative

* 2 -- end of stream; offset is usually negative

Some operating systems / file systems could provide additional

values.

Return an int indicating the new absolute position.

"""

self._unsupported("seek")

def tell(self):

"""Return an int indicating the current stream position."""

return self.seek(0, 1)

def truncate(self, pos=None):

"""Truncate file to size bytes.

Size defaults to the current IO position as reported by tell().

Return the new size.

"""

self._unsupported("truncate")

### Flush and close ###

def flush(self):

"""Flush write buffers, if applicable.

This is not implemented for read-only and non-blocking streams.

"""

self._checkClosed()

# XXX Should this return the number of bytes written???

__closed = False

def close(self):

"""Flush and close the IO object.

This method has no effect if the file is already closed.

"""

if not self.__closed:

try:

self.flush()

finally:

self.__closed = True

def __del__(self):

"""Destructor. Calls close()."""

try:

closed = self.closed

except AttributeError:

# If getting closed fails, then the object is probably

# in an unusable state, so ignore.

return

if closed:

return

if dealloc_warn := getattr(self, "_dealloc_warn", None):

dealloc_warn(self)

# If close() fails, the caller logs the exception with

# sys.unraisablehook. close() must be called at the end at __del__().

self.close()

### Inquiries ###

def seekable(self):

"""Return a bool indicating whether object supports random access.

If False, seek(), tell() and truncate() will raise OSError.

This method may need to do a test seek().

"""

return False

def _checkSeekable(self, msg=None):

"""Internal: raise UnsupportedOperation if file is not seekable

"""

if not self.seekable():

raise UnsupportedOperation("File or stream is not seekable."

if msg is None else msg)

def readable(self):

"""Return a bool indicating whether object was opened for reading.

If False, read() will raise OSError.

"""

return False

def _checkReadable(self, msg=None):

"""Internal: raise UnsupportedOperation if file is not readable

"""

if not self.readable():

raise UnsupportedOperation("File or stream is not readable."

if msg is None else msg)

def writable(self):

"""Return a bool indicating whether object was opened for writing.

If False, write() and truncate() will raise OSError.

"""

return False

def _checkWritable(self, msg=None):

"""Internal: raise UnsupportedOperation if file is not writable

"""

if not self.writable():

raise UnsupportedOperation("File or stream is not writable."

if msg is None else msg)

@property

def closed(self):

"""closed: bool. True iff the file has been closed.

For backwards compatibility, this is a property, not a predicate.

"""

return self.__closed

def _checkClosed(self, msg=None):

"""Internal: raise a ValueError if file is closed

"""

if self.closed:

raise ValueError("I/O operation on closed file."

if msg is None else msg)

### Context manager ###

def __enter__(self): # That's a forward reference

"""Context management protocol. Returns self (an instance of IOBase)."""

self._checkClosed()

return self

def __exit__(self, *args):

"""Context management protocol. Calls close()"""

self.close()

### Lower-level APIs ###

# XXX Should these be present even if unimplemented?

def fileno(self):

"""Returns underlying file descriptor (an int) if one exists.

An OSError is raised if the IO object does not use a file

descriptor.

"""

self._unsupported("fileno")

def isatty(self):

"""Return a bool indicating whether this is an 'interactive' stream.

Return False if it can't be determined.

"""

self._checkClosed()

return False

### Readline[s] and writelines ###

def readline(self, size=-1):

r"""Read and return a line of bytes from the stream.

If size is specified, at most size bytes will be read.

Size should be an int.

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.

"""

# For backwards compatibility, a (slowish) readline().

if hasattr(self, "peek"):

def nreadahead():

readahead = self.peek(1)

if not readahead:

return 1

n = (readahead.find(b"\n") + 1) or len(readahead)

if size >= 0:

n = min(n, size)

return n

else:

def nreadahead():

return 1

if size is None:

size = -1

else:

try:

size_index = size.__index__

except AttributeError:

raise TypeError(f"{size!r} is not an integer")

else:

size = size_index()

res = bytearray()

while size < 0 or len(res) < size:

b = self.read(nreadahead())

if not b:

break

res += b

if res.endswith(b"\n"):

break

return bytes(res)

def __iter__(self):

self._checkClosed()

return self

def __next__(self):

line = self.readline()

if not line:

raise StopIteration

return line

def readlines(self, hint=None):

"""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.

"""

if hint is None or hint <= 0:

return list(self)

n = 0

lines = []

for line in self:

lines.append(line)

n += len(line)

if n >= hint:

break

return lines

def writelines(self, 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.

"""

self._checkClosed()

for line in lines:

self.write(line)

io.IOBase.register(IOBase)

class RawIOBase(IOBase):

"""Base class for raw binary I/O."""

# The read() method is implemented by calling readinto(); derived

# classes that want to support read() only need to implement

# readinto() as a primitive operation. In general, readinto() can be

# more efficient than read().

# (It would be tempting to also provide an implementation of

# readinto() in terms of read(), in case the latter is a more suitable

# primitive operation, but that would lead to nasty recursion in case

# a subclass doesn't implement either.)

def read(self, size=-1):

"""Read and return up to size bytes, where size is an int.

Returns an empty bytes object on EOF, or None if the object is

set not to block and has no data to read.

"""

if size is None:

size = -1

if size < 0:

return self.readall()

b = bytearray(size.__index__())

n = self.readinto(b)

if n is None:

return None

if n < 0 or n > len(b):

raise ValueError(f"readinto returned {n} outside buffer size {len(b)}")

del b[n:]

return bytes(b)

def readall(self):

"""Read until EOF, using multiple read() call."""

res = bytearray()

while data := self.read(DEFAULT_BUFFER_SIZE):

res += data

if res:

return bytes(res)

else:

# b'' or None

return data

def readinto(self, b):

"""Read bytes into a pre-allocated bytes-like object b.

Returns an int representing the number of bytes read (0 for EOF), or

None if the object is set not to block and has no data to read.

"""

self._unsupported("readinto")

def write(self, b):

"""Write the given buffer to the IO stream.

Returns the number of bytes written, which may be less than the

length of b in bytes.

"""

self._unsupported("write")

io.RawIOBase.register(RawIOBase)

class BufferedIOBase(IOBase):

"""Base class for buffered IO objects.

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.

"""

def read(self, size=-1):

"""Read and return up to size bytes, where size is an int.

If the argument is omitted, None, or negative, reads and

returns all data until 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 (XXX and for pipes?), at most one raw

read will be issued, and a short result does not imply that

EOF is imminent.

Returns an empty bytes array on EOF.

Raises BlockingIOError if the underlying raw stream has no

data at the moment.

"""

self._unsupported("read")

def read1(self, size=-1):

"""Read up to size bytes with at most one read() system call,

where size is an int.

"""

self._unsupported("read1")

def readinto(self, b):

"""Read bytes into a pre-allocated bytes-like object b.

Like read(), this may issue multiple reads to the underlying raw

stream, unless the latter is 'interactive'.

Returns an int representing the number of bytes read (0 for EOF).

Raises BlockingIOError if the underlying raw stream has no

data at the moment.

"""

return self._readinto(b, read1=False)

def readinto1(self, b):

"""Read bytes into buffer *b*, using at most one system call

Returns an int representing the number of bytes read (0 for EOF).

Raises BlockingIOError if the underlying raw stream has no

data at the moment.

"""

return self._readinto(b, read1=True)

def _readinto(self, b, read1):

if not isinstance(b, memoryview):

b = memoryview(b)

b = b.cast('B')

if read1:

data = self.read1(len(b))

else:

data = self.read(len(b))

n = len(data)

b[:n] = data

return n

def write(self, b):

"""Write the given bytes buffer to the IO stream.

Return the number of bytes written, which is always the length of b

in bytes.

Raises BlockingIOError if the buffer is full and the

underlying raw stream cannot accept more data at the moment.

"""

self._unsupported("write")

def detach(self):

"""

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.

"""

self._unsupported("detach")

io.BufferedIOBase.register(BufferedIOBase)

class _BufferedIOMixin(BufferedIOBase):

"""A mixin implementation of BufferedIOBase with an underlying raw stream.

This passes most requests on to the underlying raw stream. It

does *not* provide implementations of read(), readinto() or

write().

"""

def __init__(self, raw):

self._raw = raw

### Positioning ###

def seek(self, pos, whence=0):

new_position = self.raw.seek(pos, whence)

if new_position < 0:

raise OSError("seek() returned an invalid position")

return new_position

def tell(self):

pos = self.raw.tell()

if pos < 0:

raise OSError("tell() returned an invalid position")

return pos

def truncate(self, pos=None):

self._checkClosed()

self._checkWritable()

# Flush the stream. We're mixing buffered I/O with lower-level I/O,

# and a flush may be necessary to synch both views of the current

# file state.

self.flush()

if pos is None:

pos = self.tell()

# XXX: Should seek() be used, instead of passing the position

# XXX directly to truncate?

return self.raw.truncate(pos)

### Flush and close ###

def flush(self):

if self.closed:

raise ValueError("flush on closed file")

self.raw.flush()

def close(self):

if self.raw is not None and not self.closed:

try:

# may raise BlockingIOError or BrokenPipeError etc

self.flush()

finally:

self.raw.close()

def detach(self):

if self.raw is None:

raise ValueError("raw stream already detached")

self.flush()

raw = self._raw

self._raw = None

return raw

### Inquiries ###

def seekable(self):

return self.raw.seekable()

@property

def raw(self):

return self._raw

@property

def closed(self):

return self.raw.closed

@property

def name(self):

return self.raw.name

@property

def mode(self):

return self.raw.mode

def __getstate__(self):

raise TypeError(f"cannot pickle {self.__class__.__name__!r} object")

def __repr__(self):

modname = self.__class__.__module__

clsname = self.__class__.__qualname__

try:

name = self.name

except AttributeError:

return "<{}.{}>".format(modname, clsname)

else:

return "<{}.{} name={!r}>".format(modname, clsname, name)

def _dealloc_warn(self, source):

if dealloc_warn := getattr(self.raw, "_dealloc_warn", None):

dealloc_warn(source)

### Lower-level APIs ###

def fileno(self):

return self.raw.fileno()

def isatty(self):

return self.raw.isatty()

class BytesIO(BufferedIOBase):

"""Buffered I/O implementation using an in-memory bytes buffer."""

# Initialize _buffer as soon as possible since it's used by __del__()

# which calls close()

_buffer = None

def __init__(self, initial_bytes=None):

buf = bytearray()

if initial_bytes is not None:

buf += initial_bytes

self._buffer = buf

self._pos = 0

def __getstate__(self):

if self.closed:

raise ValueError("__getstate__ on closed file")

return self.__dict__.copy()

def getvalue(self):

"""Return the bytes value (contents) of the buffer

"""

if self.closed:

raise ValueError("getvalue on closed file")

return bytes(self._buffer)

def getbuffer(self):

"""Return a readable and writable view of the buffer.

"""

if self.closed:

raise ValueError("getbuffer on closed file")

return memoryview(self._buffer)

def close(self):

if self._buffer is not None:

self._buffer.clear()

super().close()

def read(self, size=-1):

if self.closed:

raise ValueError("read from closed file")

if size is None:

size = -1

else:

try:

size_index = size.__index__

except AttributeError:

raise TypeError(f"{size!r} is not an integer")

else:

size = size_index()

if size < 0:

size = len(self._buffer)

if len(self._buffer) <= self._pos:

return b""

newpos = min(len(self._buffer), self._pos + size)

b = self._buffer[self._pos : newpos]

self._pos = newpos

return bytes(b)

def read1(self, size=-1):

"""This is the same as read.

"""

return self.read(size)

def write(self, b):

if isinstance(b, str):

raise TypeError("can't write str to binary stream")

with memoryview(b) as view:

if self.closed:

raise ValueError("write to closed file")

n = view.nbytes # Size of any bytes-like object

if n == 0:

return 0

pos = self._pos

if pos > len(self._buffer):

# Pad buffer to pos with null bytes.

self._buffer.resize(pos)

self._buffer[pos:pos + n] = view

self._pos += n

return n

def seek(self, pos, whence=0):

if self.closed:

raise ValueError("seek on closed file")

try:

pos_index = pos.__index__

except AttributeError:

raise TypeError(f"{pos!r} is not an integer")

else:

pos = pos_index()

if whence == 0:

if pos < 0:

raise ValueError("negative seek position %r" % (pos,))

self._pos = pos

elif whence == 1:

self._pos = max(0, self._pos + pos)

elif whence == 2:

self._pos = max(0, len(self._buffer) + pos)

else:

raise ValueError("unsupported whence value")

return self._pos

def tell(self):

if self.closed:

raise ValueError("tell on closed file")

return self._pos

def truncate(self, pos=None):

if self.closed:

raise ValueError("truncate on closed file")

if pos is None:

pos = self._pos

else:

try:

pos_index = pos.__index__

except AttributeError:

raise TypeError(f"{pos!r} is not an integer")

else:

pos = pos_index()

if pos < 0:

raise ValueError("negative truncate position %r" % (pos,))

del self._buffer[pos:]

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