有几个` `try`s‘s in `’s

Question

我有几个可能的文件可以保存我的数据；它们可以以不同的方式压缩，所以要打开它们，我需要使用file()、gzip.GzipFile()和其他也返回文件对象(支持with接口)的文件。

我想尝试每一个，直到其中一个成功打开，这样我就可以这样做了

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"

如果我有几十种可能的压缩变体，这显然是不可行的。(嵌套会越来越深，代码看起来总是非常相似。)

有没有更好的方式来表达这一点？

编辑:如果可能的话，我希望在process(f)中意外地捕捉到异常时，将process(f)从try/except中剔除。

Answer 1

我会编写一个自定义的上下文管理器：

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 2

是的，你可以把你的所有变体放在一个列表中，然后尝试它们，直到其中一个可以工作，这样就取消了你的代码的嵌套：

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 3

这样行得通吗：

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"

希望这能有所帮助