pickle.dumps在每次调用时返回不同的输出

Question

I have a simple python script which pickles an object and prints it.

import pickle

o = {'first':1,'second':2,'third':3,'ls':[1,2,3]}
d = pickle.dumps(o) 
print(d)

Following are the outputs I get when i execute the same script multiple times:

• b'\\x80\\x03}q\\x00(X\\x05\\x00\\x00\\x00firstq\\x01K\\x01X\\x05\\x00\\x00\\x00thirdq\\x02K\\x03X\\x06\\x00\\x00\\x00secondq\\x03K\\x02X\\x02\\x00\\x00\\x00lsq\\x04]q\\x05(K\\x01K\\x02K\\x03eu.'

• b'\\x80\\x03}q\\x00(X\\x05\\x00\\x00\\x00thirdq\\x01K\\x03X\\x02\\x00\\x00\\x00lsq\\x02]q\\x03(K\\x01K\\x02K\\x03eX\\x05\\x00\\x00\\x00firstq\\x04K\\x01X\\x06\\x00\\x00\\x00secondq\\x05K\\x02u.'

• b'\\x80\\x03}q\\x00(X\\x05\\x00\\x00\\x00firstq\\x01K\\x01X\\x06\\x00\\x00\\x00secondq\\x02K\\x02X\\x02\\x00\\x00\\x00lsq\\x03]q\\x04(K\\x01K\\x02K\\x03eX\\x05\\x00\\x00\\x00thirdq\\x05K\\x03u.'

• b'\\x80\\x03}q\\x00(X\\x05\\x00\\x00\\x00thirdq\\x01K\\x03X\\x05\\x00\\x00\\x00firstq\\x02K\\x01X\\x02\\x00\\x00\\x00lsq\\x03]q\\x04(K\\x01K\\x02K\\x03eX\\x06\\x00\\x00\\x00secondq\\x05K\\x02u.'

Is it just a difference in ordering of the properties of the object or is there more to it?

Answer 1

In Python 3, dictionary order is dependent on hash randomisation. Each time you start your interpreter, a different, random hash seed is used. If you were to print the dictionary, you'd see the different ordering too:

$ bin/python -c "o = {'first':1,'second':2,'third':3,'ls':[1,2,3]}; print(o)"
{'first': 1, 'ls': [1, 2, 3], 'second': 2, 'third': 3}
$ bin/python -c "o = {'first':1,'second':2,'third':3,'ls':[1,2,3]}; print(o)"
{'ls': [1, 2, 3], 'third': 3, 'first': 1, 'second': 2}
$ bin/python -c "o = {'first':1,'second':2,'third':3,'ls':[1,2,3]}; print(o)"
{'second': 2, 'ls': [1, 2, 3], 'third': 3, 'first': 1}

Python uses a random seed to prevent certain types of Denial of Service attacks against programs parsing incoming user data into dictionaries, such as web servers; such an attack could otherwise predict when two strings would cause a hash collision in a dictionary and feed Python values that do nothing but create collisions, slowing down a Python program to a crawl.

You can set the seed to a fixed value with the PYTHONHASHSEED environment variable , or you can disable hash randomisation altogether:

The integer must be a decimal number in the range [0,4294967295]. Specifying the value 0 will disable hash randomization.

$ PYTHONHASHSEED=0 bin/python -c "o = {'first':1,'second':2,'third':3,'ls':[1,2,3]}; print(o)"
{'third': 3, 'ls': [1, 2, 3], 'first': 1, 'second': 2}
$ PYTHONHASHSEED=0 bin/python -c "o = {'first':1,'second':2,'third':3,'ls':[1,2,3]}; print(o)"
{'third': 3, 'ls': [1, 2, 3], 'first': 1, 'second': 2}

Also see: Why is the order in dictionaries and sets arbitrary?