pickle.load() and pickle.dump() take a file object as an argument. That file object must be opened in binary mode ("rb" or "wb"). If it's not, pickle raises a confusing error message.

When pickle.dump()'ing to a file opened in "w" mode instead of "wb" mode it'll raise a TypeError:


TypeError: write() argument must be str, not bytes


I thought it might be getting this because I was pickling bytes objects. 

For pickle.load()'ing from a file opened in "r" mode instead of "rb" mode, it'll raise a UnicodeDecodeError


UnicodeDecodeError: 'utf-8' codec can't decode byte 0x80 in position 0: invalid start byte


There are a couple pages on the internet (just search "pickle" followed by either error message) with people being confused about this. 

pickle should catch these errors and report something more useful.