几个`与`s在`尝试`s

Question

我有几个可能保存我的数据的文件; 它们可以以不同的方式压缩,因此要打开它们我需要使用file(),gzip.GzipFile()而其他也返回文件对象(支持with接口).

我想尝试每一个,直到一个成功开放,所以我可以做类似的事情

try:
  with gzip.GzipFile(fn + '.gz') as f:
    result = process(f)
except (IOError, MaybeSomeGzipExceptions):
  try:
    with xCompressLib.xCompressFile(fn + '.x') as f:
      result = process(f)
  except (IOError, MaybeSomeXCompressExceptions):
    try:
      with file(fn) as f:
        result = process(f)
    except IOError:
      result = "some default value"

如果我有几十个可能的压缩变体,这显然是不可行的.(嵌套将越来越深,代码看起来总是非常相似.)

有没有更好的拼写方法？

编辑:如果可能的话,我希望process(f)除了try/except以外,以避免意外捕获在中提出的异常process(f).

Answer 1

是的,您可以将所有变体放在列表中并尝试它们直到其中一个工作,从而取消嵌套代码:

def process_gzip(fn):
    with gzip.GzipFile(fn + '.gz') as f:
        return process(f)

def process_xlib(fn):
    with xCompressLib.xCompressFile(fn + '.x') as f:
        return process(f)

def process_builtin(fn):
    with file(fn) as f:
        return process(f)

process_funcs = [process_gzip, process_xlib, process_builtin]

#processing code:

for process_f in process_funcs:
    try:
        result = process_f(fn)
        break
    except IOError:
        #error reading the file, keep going
        continue
    except:
        #processing error, re-raise the exception
        raise

或者,为了减少代码量,您可以创建一个process_func工厂,因为它们都具有相同的形式:

def make_process_func(constructor, filename_transform):
    with constructor(filename_transform) as f:
        return process(f)

process_funcs = [
    make_process_func(gzip.GzipFile, lambda fn: fn + '.gz'),
    make_process_func(xCompressLib.xCompressFile, lambda fn: fn + '.x'),
    make_process_func(file, lambda fn: fn),
]

Answer 2

我写了一个自定义上下文管理器:

from contextlib import contextmanager

filetypes = [('.gz', gzip.GzipFile, (IOError, MaybeSomeGzipExceptions)), 
             ('.x', xCompressLib.xCompressFile, (IOError, MaybeSomeXCompressExceptions))]

@contextmanager
def open_compressed(fn):
    f = None
    try:
        for ext, cls, exs in filetypes:
            try:
                f = cls(fn + ext)
            except exs:
                pass
            else:
                break
        yield f
    finally:
        if f is not None:
            f.close()

with open_compressed(fn) as f:
    result = "some default value" if f is None else process(f)

或者可能只是一个返回上下文管理器的函数:

filetypes = [('.gz', gzip.GzipFile, (IOError, MaybeSomeGzipExceptions)), 
             ('.x', xCompressLib.xCompressFile, (IOError, MaybeSomeXCompressExceptions))]

class UnknownCompressionFormat(Exception):
    pass

def open_compressed(fn):
    for ext, cls, exs in filetypes:
        try:
            return cls(fn + ext)
        except exs:
            pass
    raise UnknownCompressionFormat

try:
    with open_compressed(fn) as f:
        result = process(f)
except UnknownCompressionFormat:
    result = "some default value"

Answer 3

这会工作:

extensions = [('.gz', gzip.GzipFile, (IOError, MaybeSomeGzipExceptions)), 
              ('.x', xCompressLib.xCompressFile, (IOError, MaybeSomeXCompressExceptions))] # and other such entries
processed = False
for ext, (compressor, errors) in extensions.iteritems():
    try:
        with compressor(fn+ext) as f:
            try:
                result = process(f)
                processed = True
                break
            except:
                raise
    except errors:
        pass
if not processed:
    result = "some default value"

希望有所帮助