Detecteing Unicode encodings

[Jason Diamond]

Christos TZOTZIOY Georgiou wrote:
> 2. wrap your file-like object in a custom object, which implements a
> pushback method and its read method returns first from the push-back
> buffer.  If you read data that you shouldn't, push them back and give
> your custom object to the StreamReader.

Thanks for the suggestion.

Instead of a pushback method, I added a peek method. Below is what I 
came up with.

-- Jason

class PeekableFile:

     def __init__(self, source):
         self.source = source
         self.buffer = None

     def peek(self, size):
         if self.buffer:
             n = len(self.buffer)
             if size > n:
                 self.buffer += self.source.read(size - n)
         else:
             self.buffer = self.source.read(size)
         return self.buffer[:size]

     def read(self, size=-1):
         if self.buffer:
             if size >= 0:
                 n = len(self.buffer)
                 if size < n:
                     s = self.buffer[:size]
                     self.buffer = self.buffer[size:]
                 elif size == n:
                     s = self.buffer
                     self.buffer = None
                 else:
                     s = self.buffer + self.source.read(size - n)
                     self.buffer = None
             else:
                 s = self.buffer + self.source.read()
                 self.buffer = None
         else:
             s = self.source.read(size)
         return s

def main():

     import StringIO
     import unittest

     class PeekableFileTests(unittest.TestCase):

         def setUp(self):
             f = StringIO.StringIO('abc')
             self.pf = PeekableFile(f)

         def testPeek0(self):
             self.failUnlessEqual(self.pf.peek(0), '')

         def testPeek1(self):
             self.failUnlessEqual(self.pf.peek(1), 'a')

         def testPeek1Read1(self):
             self.failUnlessEqual(self.pf.peek(1), 'a')
             self.failUnlessEqual(self.pf.read(1), 'a')

         def testPeek1Read2(self):
             self.failUnlessEqual(self.pf.peek(1), 'a')
             self.failUnlessEqual(self.pf.read(2), 'ab')

         def testPeek1ReadAll(self):
             self.failUnlessEqual(self.pf.peek(1), 'a')
             self.failUnlessEqual(self.pf.read(), 'abc')

         def testPeek1Read1Read1(self):
             self.failUnlessEqual(self.pf.peek(1), 'a')
             self.failUnlessEqual(self.pf.read(1), 'a')
             self.failUnlessEqual(self.pf.read(1), 'b')

         def testPeek1Read1ReadAll(self):
             self.failUnlessEqual(self.pf.peek(1), 'a')
             self.failUnlessEqual(self.pf.read(1), 'a')
             self.failUnlessEqual(self.pf.read(), 'bc')

         def testPeek1Peek1(self):
             self.failUnlessEqual(self.pf.peek(1), 'a')
             self.failUnlessEqual(self.pf.peek(1), 'a')

         def testPeek1Peek2(self):
             self.failUnlessEqual(self.pf.peek(1), 'a')
             self.failUnlessEqual(self.pf.peek(2), 'ab')

         def testPeek2Peek1(self):
             self.failUnlessEqual(self.pf.peek(2), 'ab')
             self.failUnlessEqual(self.pf.peek(1), 'a')

         def testPeek2Read1Peek1(self):
             self.failUnlessEqual(self.pf.peek(2), 'ab')
             self.failUnlessEqual(self.pf.read(1), 'a')
             self.failUnlessEqual(self.pf.peek(1), 'b')

         def testRead0(self):
             self.failUnlessEqual(self.pf.read(0), '')

         def testRead1(self):
             self.failUnlessEqual(self.pf.read(1), 'a')

         def testReadAll(self):
             self.failUnlessEqual(self.pf.read(), 'abc')

         def testRead1Peek1(self):
             self.failUnlessEqual(self.pf.read(1), 'a')
             self.failUnlessEqual(self.pf.peek(1), 'b')

         def testReadAllPeek1(self):
             self.failUnlessEqual(self.pf.read(), 'abc')
             self.failUnlessEqual(self.pf.peek(1), '')

     unittest.TextTestRunner().run(unittest.makeSuite(PeekableFileTests))

if __name__ == '__main__':
     main()