reading zipfile; problem using raw buffer

[Sells, Fred]

I'm tring to unzip a buffer that is uploaded to django/python.  I can
unzip the file in batch mode just fine, but when I get the buffer I get
a "BadZipfile exception.  I wrote this snippet to try to isolate the
issue but I don't understand what's going on.  I'm guessing that I'm
losing some header/trailer somewhere?

def unittestZipfile(filename):
    buffer = ''
    f = open(filename)
    for i in range(22):
        block = f.read()
        if len(block) == 0: 
            break
        else:
            buffer += block

    print len(buffer)
    tmp = open('tmp.zip', 'w')
    tmp.write(buffer)
    tmp.close()
    zf = zipfile.ZipFile('tmp.zip')
    print dir(zf)
    for name in zf.namelist():
        print name
        print zf.read(name)
____________________________________________________________
2.6.6 (r266:84297, Aug 24 2010, 18:46:32) [MSC v.1500 32 bit (Intel)]
Traceback (most recent call last):
  File
"C:\all\projects\AccMDS30Server\mds30\app\uploaders\xmitzipfile.py",
line 162, in <module>
    unittestZipfile('wk1live7.8to7.11.zip')    
  File
"C:\all\projects\AccMDS30Server\mds30\app\uploaders\xmitzipfile.py",
line 146, in unittestZipfile
    print zf.read(name)
  File "C:\alltools\python26\lib\zipfile.py", line 837, in read
    return self.open(name, "r", pwd).read()
  File "C:\alltools\python26\lib\zipfile.py", line 867, in open
    raise BadZipfile, "Bad magic number for file header"
zipfile.BadZipfile: Bad magic number for file header