I'm working on a "TempFile" class that stores the data in memory until
it gets larger than a specified threshold (as per PEP 42). Whilst
trying to implement it, I've come across some strange behaviour. Can
anyone explain this?
The test case at the bottom starts a TempFile at size 50 and prints its
type. It then increases the size to the threshold at which point
"self" is changed to being a TemporaryFile. It seems that the next
call correctly uses the write() method of TemporaryFile (since we don't
see "changing type" in the output). However, type(tmp) still equals
TempFile. Not only that, tmp can still access the method dummy() that
exists only in TempFile.
#!/usr/bin/env python
from StringIO import StringIO
import tempfile
class TempFile(StringIO, object):
"""A temporary file implementation that uses memory unless
either capacity is breached or fileno is requested, at which
point a real temporary file will be created and the relevant
details returned
"""
def __init__(self, buffer, capacity):
"""Creates a TempFile object containing the specified buffer.
If capacity is specified, we use a real temporary file once the
file gets larger than that size. Otherwise, the data is stored
in memory.
"""
self.capacity = capacity
if len(buffer) > capacity:
self = tempfile.TemporaryFile()
self.write(buffer)
else:
super(TempFile, self).__init__(buffer)
def dummy(self):
pass
def write(self, str):
self.seek(0, 2) # find end of file
if((self.tell() + len(str)) >= self.capacity):
print "changing type"
flo = tempfile.TemporaryFile()
flo.write(self.getvalue())
self = flo
print type(self)
else:
super(TempFile, self).write(str)
print "testing tempfile:"
tmp = TempFile("", 100)
ten_chars = "1234567890"
tmp.write(ten_chars * 5)
tmp.dummy()
print "tmp < 100: " + str(type(tmp))
tmp.write(ten_chars * 5)
tmp.dummy()
print "tmp == 100: " + str(type(tmp))
tmp.write("the last straw")
tmp.dummy()
print "tmp > 100: " + str(type(tmp))
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