Why doesn't the Shelve module work in Python 3?

Question

Python

Good afternoon. There is a piece of code:

import shelve

data = shelve.open("quiz")
data["quiz1"] = {"theme" : None}
data["quiz1"]["theme"] = "Cinematograph"
print(data["quiz1"]["theme"])
data.close()

I am writing in Python 3.5.1 . With the print()
function , the interpreter prints None , even though the library had assigned a value to this key in the line above. What am I doing wrong? It is worth adding that I study the language from the book "Programming in Python" by M. Dawson and use the module as he explained.

Answer 1

Look. data is a file handle that mimics the behavior of a dictionary. It has a __setattr__ method that is called directly when you write the "square brackets" operator with equality. In your case:
data['key'] = {'this': 'is dict'}
The datatype of data is shelve
The datatype of data['key'] is a dictionary
When you write this:
data["quiz1"][" theme"] = "Cinematograph"
the __setattr__ method is called on the data["quiz1"] dictionary
and it has nothing to do with shelve. Therefore, in order to do what you want, you need to rewrite it somehow like this:

import shelve

data = shelve.open("quiz")
data["quiz1"] = {"theme" : None}
tmp = data["quiz1"]
tmp["theme"] = "Cinematograph"
data["quiz1"] = tmp
print(data["quiz1"]["theme"])
data.close()

Or open shelve with the key writeback=True: data = shelve.open('quiz', writeback=True)
But writeback must be used carefully:
By the way, this is written in the documentation.