python中type 5.5_5. 内置类型

5.9.File Objects¶

File objects are implemented using C’s stdio package and can be

created with the built-in open() function. File

objects are also returned by some other built-in functions and methods,

such as os.popen() and os.fdopen() and the makefile()

method of socket objects. Temporary files can be created using the

tempfile module, and high-level file operations such as copying,

moving, and deleting files and directories can be achieved with the

shutil module.

When a file operation fails for an I/O-related reason, the exception

IOError is raised. This includes situations where the operation is not

defined for some reason, like seek() on a tty device or writing a file

opened for reading.

Files have the following methods:

file.close()¶

Close the file. A closed file cannot be read or written any more. Any operation

which requires that the file be open will raise a ValueError after the

file has been closed. Calling close() more than once is allowed.

As of Python 2.5, you can avoid having to call this method explicitly if you use

the with statement. For example, the following code will

automatically close f when the with block is exited:

from __future__ import with_statement # This isn't required in Python 2.6

with open("hello.txt") as f:

for line in f:

print line,

In older versions of Python, you would have needed to do this to get the same

effect:

f = open("hello.txt")

try:

for line in f:

print line,

finally:

f.close()

注解

Not all “file-like” types in Python support use as a context manager for the

with statement. If your code is intended to work with any file-like

object, you can use the function contextlib.closing() instead of using

the object directly.

file.flush()¶

Flush the internal buffer, like stdio’s fflush(). This may be a

no-op on some file-like objects.

注解

flush() does not necessarily write the file’s data to disk. Use

flush() followed by os.fsync() to ensure this behavior.

file.fileno()¶

Return the integer “file descriptor” that is used by the underlying

implementation to request I/O operations from the operating system. This can be

useful for other, lower level interfaces that use file descriptors, such as the

fcntl module or os.read() and friends.

注解

File-like objects which do not have a real file descriptor should not provide

this method!

file.isatty()¶

Return True if the file is connected to a tty(-like) device, else False.

注解

If a file-like object is not associated with a real file, this method should

not be implemented.

file.next()¶

A file object is its own iterator, for example iter(f) returns f (unless

f is closed). When a file is used as an iterator, typically in a

for loop (for example, for line in f: print line.strip()), the

next() method is called repeatedly. This method returns the next input

line, or raises StopIteration when EOF is hit when the file is open for

reading (behavior is undefined when the file is open for writing). In order to

make a for loop the most efficient way of looping over the lines of a

file (a very common operation), the next() method uses a hidden read-ahead

buffer. As a consequence of using a read-ahead buffer, combining next()

with other file methods (like readline()) does not work right. However,

using seek() to reposition the file to an absolute position will flush the

read-ahead buffer.

2.3 新版功能.

file.read([size])¶

Read at most size bytes from the file (less if the read hits EOF before

obtaining size bytes). If the size argument is negative or omitted, read

all data until EOF is reached. The bytes are returned as a string object. An

empty string is returned when EOF is encountered immediately. (For certain

files, like ttys, it makes sense to continue reading after an EOF is hit.) Note

that this method may call the underlying C function fread() more than

once in an effort to acquire as close to size bytes as possible. Also note

that when in non-blocking mode, less data than was requested may be

returned, even if no size parameter was given.

注解

This function is simply a wrapper for the underlying

fread() C function, and will behave the same in corner cases,

such as whether the EOF value is cached.

file.readline([size])¶

Read one entire line from the file. A trailing newline character is kept in

the string (but may be absent when a file ends with an incomplete line). 6

If the size argument is present and non-negative, it is a maximum byte

count (including the trailing newline) and an incomplete line may be

returned. When size is not 0, an empty string is returned only when EOF

is encountered immediately.

注解

Unlike stdio’s fgets(), the returned string contains null characters

('\0') if they occurred in the input.

file.readlines([sizehint])¶

Read until EOF using readline() and return a list containing the lines

thus read. If the optional sizehint argument is present, instead of

reading up to EOF, whole lines totalling approximately sizehint bytes

(possibly after rounding up to an internal buffer size) are read. Objects

implementing a file-like interface may choose to ignore sizehint if it

cannot be implemented, or cannot be implemented efficiently.

file.xreadlines()¶

This method returns the same thing as iter(f).

2.1 新版功能.

2.3 版后已移除:Use for line in file instead.

file.seek(offset[, whence])¶

Set the file’s current position, like stdio’s fseek(). The whence

argument is optional and defaults to os.SEEK_SET or 0 (absolute file

positioning); other values are os.SEEK_CUR or 1 (seek relative to the

current position) and os.SEEK_END or 2 (seek relative to the file’s

end). There is no return value.

For example, f.seek(2, os.SEEK_CUR) advances the position by two and

f.seek(-3, os.SEEK_END) sets the position to the third to last.

Note that if the file is opened for appending

(mode 'a' or 'a+'), any seek() operations will be undone at the

next write. If the file is only opened for writing in append mode (mode

'a'), this method is essentially a no-op, but it remains useful for files

opened in append mode with reading enabled (mode 'a+'). If the file is

opened in text mode (without 'b'), only offsets returned by tell() are

legal. Use of other offsets causes undefined behavior.

Note that not all file objects are seekable.

在 2.6 版更改:Passing float values as offset has been deprecated.

file.tell()¶

Return the file’s current position, like stdio’s ftell().

注解

On Windows, tell() can return illegal values (after an fgets())

when reading files with Unix-style line-endings. Use binary mode ('rb') to

circumvent this problem.

file.truncate([size])¶

Truncate the file’s size. If the optional size argument is present, the file

is truncated to (at most) that size. The size defaults to the current position.

The current file position is not changed. Note that if a specified size exceeds

the file’s current size, the result is platform-dependent: possibilities

include that the file may remain unchanged, increase to the specified size as if

zero-filled, or increase to the specified size with undefined new content.

Availability: Windows, many Unix variants.

file.write(str)¶

Write a string to the file. There is no return value. Due to buffering, the

string may not actually show up in the file until the flush() or

close() method is called.

file.writelines(sequence)¶

Write a sequence of strings to the file. The sequence can be any iterable

object producing strings, typically a list of strings. There is no return value.

(The name is intended to match readlines(); writelines() does not

add line separators.)

Files support the iterator protocol. Each iteration returns the same result as

readline(), and iteration ends when the readline() method returns

an empty string.

File objects also offer a number of other interesting attributes. These are not

required for file-like objects, but should be implemented if they make sense for

the particular object.

file.closed¶

bool indicating the current state of the file object. This is a read-only

attribute; the close() method changes the value. It may not be available

on all file-like objects.

file.encoding¶

The encoding that this file uses. When Unicode strings are written to a file,

they will be converted to byte strings using this encoding. In addition, when

the file is connected to a terminal, the attribute gives the encoding that the

terminal is likely to use (that information might be incorrect if the user has

misconfigured the terminal). The attribute is read-only and may not be present

on all file-like objects. It may also be None, in which case the file uses

the system default encoding for converting Unicode strings.

2.3 新版功能.

file.errors¶

The Unicode error handler used along with the encoding.

2.6 新版功能.

file.mode¶

The I/O mode for the file. If the file was created using the open()

built-in function, this will be the value of the mode parameter. This is a

read-only attribute and may not be present on all file-like objects.

file.name¶

If the file object was created using open(), the name of the file.

Otherwise, some string that indicates the source of the file object, of the

form <...>. This is a read-only attribute and may not be present on all

file-like objects.

file.newlines¶

If Python was built with universal newlines enabled (the default) this

read-only attribute exists, and for files opened in universal newline read

mode it keeps track of the types of newlines encountered while reading the

file. The values it can take are '\r', '\n', '\r\n', None

(unknown, no newlines read yet) or a tuple containing all the newline types

seen, to indicate that multiple newline conventions were encountered. For

files not opened in universal newlines read mode the value of this attribute

will be None.

file.softspace¶

Boolean that indicates whether a space character needs to be printed before

another value when using the print statement. Classes that are trying

to simulate a file object should also have a writable softspace

attribute, which should be initialized to zero. This will be automatic for most

classes implemented in Python (care may be needed for objects that override

attribute access); types implemented in C will have to provide a writable

softspace attribute.

注解

This attribute is not used to control the print statement, but to

allow the implementation of print to keep track of its internal

state.