Store a variable permanently

[Steven D'Aprano]

On Mon, 11 Mar 2013 11:19:49 +0100, Jean-Michel Pichavant wrote:

[...]
> While your point about security is fair, the others aren't. Pickle uses
> by default an ascii representation of the data, it's readable and
> writeable.
> 
> import pickle
> a = 758
> pickle.dump(a, open('test.pickle', 'w')) 
> !cat test.pickle
> I758
> .


What is that? It's not Python code, !cat test.pickle gives a syntax error.


By the way, you can dump pickles directly to a string, which may be more 
convenient for demonstration purposes:

py> import pickle
py> pickle.dumps(758)
'I758\n.'


I take your point that a pickle of a simple int is relatively readable, 
although it does require care when editing. If you drop the dot, or the 
newline, or change the I to lowercase, or even merely add a space after 
the dot, bad things happen.

But yes, I will concede that a single pickled int is relatively readable. 
But that certainly isn't always the case:

py> pickle.dumps([])
'(lp0\n.'
py> pickle.dumps([None])
'(lp0\nNa.'


For even a *slightly* more complex example, the pickle turns into noise.



> I don't see how 1 line of code (+ the import) can be overkill versus the
> dozen untested lines you provide (I'm sure it's working, my point being
> pickle has already been tested). 

Pickle is a big module, over 1400 lines, capable of serialising almost 
anything. It's a big, powerful hammer for cracking armour-plated 
coconuts. But a single int is pretty much a peanut. Compare pickle's 1400 
lines with the dozen or so lines I provided. That is all that I meant by 
"overkill".


> More importantly, if the code evolve
> and you need to store 2 integers, or a tuple or anything else that is
> pickable, it costs you 0 dev if you're using pickle.


Sure. And once you move beyond a single value, the ability to call pickle 
"human readable and writable" decreases rapidly. Without using pickle, 
can you tell what this represents?

((dp0
S'y'
p1
I3
sS'x'
p2
I2
s(lp3
S'a'
p4
aS'b'
p5
aI23
tp6
.




-- 
Steven