Boolean value of generators

Tim Chase python.list at
Thu Oct 14 21:13:30 CEST 2010

On 10/14/10 12:53, Paul Rubin wrote:
> Carl Banks<pavlovevidence at>  writes:
>> In general, the only way to test if a generator is empty is to try to
>> consume an item.  (It's possible to write an iterator that consumes an
>> item and caches it to be returned on the next next(), and whose
>> boolean status indicates if there's an item left. ...)
> I remember thinking that Python would be better off if all generators
> automatically cached an item, so you could test for emptiness, look
> ahead at the next item without consuming it, etc.  This might have been
> a good change to make in Python 3.0 (it would have broken compatibility
> with 2.x) but it's too late now.

Generators can do dangerous things...I'm not sure I'd *want* to 
have Python implicitly cache generators without an explicit 
wrapper to request it:

  import os
  from fnmatch import fnmatch

  def delete_info(root, pattern):
    for path, dirs, files in os.walk(root):
      for fname in files:
        if fnmatch(fname, pattern):
          full_path = os.path.join(path, fname)
          info = gather_info(full_path)
          yield full_path, info

  location = '/'
  user_globspec = '*.*'
  deleter = delete_info(location, user_globspec)
  if some_user_condition_determined_after_generator_creation:
    for path, info in deleter:
      report(path, info)


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