Adapt an iterator to behave like a file-like object in Python

I have a generator producing a list of strings. Is there a utility/adapter in Python that could make it look like a file?

For example,

>>> def str_fn():
...     for c in 'a', 'b', 'c':
...         yield c * 3
... 
>>> for s in str_fn():
...     print s
... 
aaa
bbb
ccc
>>> stream = some_magic_adaptor(str_fn())
>>> while True:
...    data = stream.read(4)
...    if not data:
...        break
...    print data
aaab
bbcc
c

Because data may be big and needs to be streamable (each fragment is a few kilobytes, the entire stream is tens of megabytes), I do not want to eagerly evaluate the whole generator before passing it to stream adaptor.

Answer

Here’s a solution that should read from your iterator in chunks.

class some_magic_adaptor:
  def __init__( self, it ):
    self.it = it
    self.next_chunk = ""
  def growChunk( self ):
    self.next_chunk = self.next_chunk + self.it.next()
  def read( self, n ):
    if self.next_chunk == None:
      return None
    try:
      while len(self.next_chunk)<n:
        self.growChunk()
      rv = self.next_chunk[:n]
      self.next_chunk = self.next_chunk[n:]
      return rv
    except StopIteration:
      rv = self.next_chunk
      self.next_chunk = None
      return rv


def str_fn():
  for c in 'a', 'b', 'c':
    yield c * 3

ff = some_magic_adaptor( str_fn() )

while True:
  data = ff.read(4)
  if not data:
    break
  print data