`嘗試`中有```

Question

我有幾個可能的文件可以容納我的數據;他們可以用不同的方式壓縮，所以要打開它們，我需要使用file(),gzip.GzipFile()和其他也返回一個文件對象（支持with接口）的其他。

我想試試它們中的每一個，直到在開放成功，所以我可以做類似

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" 

這顯然不是萬一我有幾十個可能的壓縮變形是可行的。 （嵌套會變得越來越深，代碼看起來非常相似。）

有沒有更好的方法來說明這一點？

編輯：如果可能，我想有process(f)出來的嘗試/除了以及以避免意外捕捉process(f)中引發的異常。

Answer 1

我會寫一個自定義的上下文管理器：

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，也許只是一個函數返回一個上下文管理器：

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，你可以把你所有的變種通過名單，並嘗試他們，直到他們的作品之一，因而未嵌套代碼：

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，減少代碼量，你可以做一個process_func工廠，因爲它們都具有相同的形式：

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

將這工作：

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" 

希望幫助