[Python编程(第4版)].(Programming.Python.4th.Edition).Mark.Lutz.文字版

But on some systems, os.open flags let us specify more advanced things like exclusive

access (O_EXCL) and nonblocking modes (O_NONBLOCK) when a file is opened. Some of

these flags are not portable across platforms (another reason to use built-in file objects

most of the time); see the library manual or run a dir(os) call on your machine for an

exhaustive list of other open flags available.

One final note here: using os.open w i t h t h e O_EXCL f l a g i s t h e m o s t p o r t a b l e w a y t o lock

files for concurrent updates or other process synchronization in Python today. We’ll

see contexts where this can matter in the next chapter, when we begin to explore

multiprocessing tools. Programs running in parallel on a server machine, for instance,

may need to lock files before performing updates, if multiple threads or processes might

attempt such updates at the same time.

Wrapping descriptors in file objects

We saw earlier how to go from file object to field descriptor with the fileno file object

method; given a descriptor, we can use os module tools for lower-level file access to

the underlying file. We can also go the other way—the os.fdopen call wraps a file de-

scriptor in a file object. Because conversions work both ways, we can generally use

either tool set—file object or os module:

>>> fdfile = os.open(r'C:\temp\spam.txt', (os.O_RDWR | os.O_BINARY))

>>> fdfile

3

>>> objfile = os.fdopen(fdfile, 'rb')

>>> objfile.read()

b'Jello stdio file\r\nHello descriptor file\n'

In fact, we can wrap a file descriptor in either a binary or text-mode file object: in text

mode, reads and writes perform the Unicode encodings and line-end translations we

studied earlier and deal in str strings instead of bytes:

C:\...\PP4E\System> python

>>> import os

>>> fdfile = os.open(r'C:\temp\spam.txt', (os.O_RDWR | os.O_BINARY))

>>> objfile = os.fdopen(fdfile, 'r')

>>> objfile.read()

'Jello stdio file\nHello descriptor file\n'

In Python 3.X, the built-in open call also accepts a file descriptor instead of a file name

string; in this mode it works much like os.fdopen, but gives you greater control—for

example, you can use additional arguments to specify a nondefault Unicode encoding

for text and suppress the default descriptor close. Really, though, os.fdopen accepts

the same extra-control arguments in 3.X, because it has been redefined to do little but

call back to the built-in open (see os.py in the standard library):

C:\...\PP4E\System> python

>>> import os

>>> fdfile = os.open(r'C:\temp\spam.txt', (os.O_RDWR | os.O_BINARY))

>>> fdfile

3

158 | Chapter 4: File and Directory Tools

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