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Python Hands on - The Pickle Problem

30 May 2011

I recently learned how to write to a file in Python, and among it how to use Object Serialization library called “pickle”.

According to Documentation it should be quite easy

pickle.dump(x, f)
x = pickle.load(f)  

What the documentation does not tell you is that you need to open the file with a “binary” flag.
So some of you might get

write() argument 1 must be str, not bytes

All you need to do is add

f = open("myFile","wb")

The “b” character, tells python to open the file in a binary mode.
Same for reading! So if you are getting

read() from the underlying stream did notreturn bytes

make sure you did

f = open("myFile","rb")

If some of you are getting

TypeError: file must have a 'write' attribute

This means you sent the argument in a wrong order. Note the signature of “dump”

def dump(obj, file, protocol=None, *, fix_imports=True):

Make sure that you first send the object, then the file.

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