[Python-Dev] suggestion: except in list comprehension (original) (raw)

Josiah Carlson jcarlson at uci.edu
Wed Apr 26 19:51:33 CEST 2006

"tomer filiba" <tomerfiliba at gmail.com> wrote:

"[" for in [if ] [except : ] "]"

Note that of the continue cases you offer, all of them are merely simple if condition (though the file example could use a better test than os.path.isfile).

[x for x in a if x.startswith("y") except AttributeError: continue]
[x for x in a if hasattr(x, 'startswith') and x.startswith("y")]

[1.0 / x for x in y except ZeroDivisionError: continue]
[1.0 / x for x in y if x != 0]

[open(filename) for filename in filelist except IOError: continue]
[open(filename) for filename in filelist if os.path.isfile(filename)]

The break case can be implemented with particular kind of instance object, though doesn't have the short-circuiting behavior...

class StopWhenFalse: def init(self): self.t = 1 def call(self, t): if not t: self.t = 0 return 0 return self.t

z = StopWhenFalse()

Assuming you create a new instance z of StopWhenFalse before doing the list comprehensions...

[x for x in a if z(hasattr(x, 'startswith') and x.startswith("y"))]
[1.0 / x for x in y if z(x != 0)]
[open(filename) for filename in filelist if z(os.path.isfile(filename))]

If you couldn't guess; -1, you can get equivalent behavior without complicating the generator expression/list comprension syntax.

More information about the Python-Dev mailing list