Usage of pickle.dump in Python

Question

I'm trying to learn how to use the pickle module in Python:

import pickle x = 123 f = open('data.txt','w') pickle.dump(x,f) 

Here's what I get:

Traceback (most recent call last):   File "D:\python\test.py", line 5, in <module>     pickle.dump(x,f) TypeError: must be str, not bytes 

However, this code works just fine:

import pickle dump = pickle.dump(123) print(dump) 

What am I doing wrong?

Answer 1

The problem is that you're opening the file in text mode. You need to use binary here:

>>> f = open('data.txt','w') >>> pickle.dump(123,f) Traceback (most recent call last):   File "<stdin>", line 1, in <module> TypeError: must be str, not bytes >>>  >>> f = open('data.txt','wb') >>> pickle.dump(123,f) >>>  

Answer 2

The write method for file-like objects, only accept a single string argument. The dumps method in the pickle module automatically casts arguments as strings, whereas the the dump method will write a pickled representation of the object to the open file. Since 123 is not a string it throws the TypeError error.

This is acknowledged in pickle.dump documentation.