Programming Python for absolute beginners: Chapter 7 Storing Complex Data

Question

I'm learning from "Python programming for the absolute beginner" and have been having fun. The book is written for Python 2.7 (i think), but I've been using Python 3 and translating the code, which is been a fun challenge.

I recently ran into a problem I'm not sure how to fix. On the section labeled: "Pickling Data and Writing It to a File" There's an example where you run the following code:

import cPickle, shelve
print "Pickling lists." variety = ["sweet", "hot", "dill"]
shape = ["whole", "spear", "chip"]
brand = ["Claussen", "Heinz", "Vlassic"]
pickle_file = open("pickles1.dat", "w")
cPickle.dump(variety, pickle_file)
cPickle.dump(shape, pickle_file)
cPickle.dump(brand, pickle_file)
pickle_file.close()

I translated it to this (for python 3)

import pickle, shelve
print ("Pickling lists.")
variety = ["sweet", "hot", "dill"]
shape = ["whole", "spear", "chip"]
brand = ["Classen", "Heinz", "Vlassic"]
pickle_file = open("pickles1.dat", "w")
pickle.dump(variety, pickle_file)
pickle.dump(shape, pickle_file)
pickle.dump(brand, pickle_file)
pickle_file.close()

BUT, i get this error/output from IDLE:

Pickling lists.
Traceback (most recent call last):
File "/Users/hypernerdcc/Documents/pickles.py", line 11, in <module>
pickle.dump(variety, pickle_file)
File
"/Library/Frameworks/Python.framework/Versions/3.1/lib/python3.1/pickle.py",
line 1345, in dump
Pickler(file, protocol, fix_imports=fix_imports).dump(obj)
TypeError: must be str, not bytes

Any ideas?

Answer 1

You are trying to write bytes, ie binary data, to a text file, which will only accept str. Change the file opening flags, it should be:

pickle_file = open("pickles1.dat", "wb")

With the b flagging it as a binary file, which then will accept bytes.

This is in fact a bug in the book. The binary flag should really be there in the Python 2 code as well.