File I/O self-WTF

Noam_Samuel

I was working on an email routing script for a relative of mine, and one of the specifications was that no duplicate emails can arrive within a specific span of time. OK, so I used a dictionary object with the keys being the mails and the values being the last time they were sent, so that if the mail was not in dupes.keys() or dupes[email] = time() - configurable_timespan, it would be OK, but that otherwise it would bounce.

Anyway, I was using pickle to make the object persistent, and I wanted to make sure it would create an empty dictionary if the file was empty. thus, the following code was created:

[code]
if dupe_file.read():
dupes = pickle.load(dupe_file)
else:
dupes = {}
[/code]

multiple hours and bunches of searches for EOFError later, I suddenly realized what was wrong.

Noam_Samuel

Arrgggh!

[code]

if dupe_file.read():

dupes = pickle.load(dupe_file)

else:

dupes = {}

[/code]

Noam_Samuel

I don't believe this!


if dupe_file.read():

dupes = pickle.load(dupe_file)

else:

dupes = {}

codeman

I don't know pickle - what's the wtf?

Noam_Samuel

Pickle creates persistant objects by serializing them and writing them to a file (and, obviously, by retrieving it).

The
WTF was that since I put a dupe_file.read() statement in the if (to
check if the file has contents), the position I was in the file was the
end of the file. Thus, any request that would have involved reading from the file would have returned an EOFError.

codeman

I'd laugh, but I've inadvertently done stuff like that myself because my mind was on frying bigger fish.

El_Foo

This is a minor wtf, or, as I prefer to call them, 'sleep-deprived wtf' because of the state you're usually in when you write code like this. I've done comparatively stupid things in the past, but one of my favourites was in a mailing on USENET (for the record, something completely different was being discussed, but some illustrative code was needed and this was part of it):

<pre>
let res = any $ zipWith ((x, y) -> x == y) xs ys
</pre>

For the uninitiated, here it is in imperative-pythonesquepseudocode:

<pre>
xs = [List of stuff]
ys = [Another list of stuff]
for x, y in zip(xs, ys):
if x == y:
return True
</pre>

Of course, both of these can be simply reduced to <pre>xs == ys</pre>

PS: will this work? Who knows if the mysterious supposed 'telligent system' will accept the <pre> tags...

Fred

@Noam Samuel said:

multiple hours and bunches of searches for EOFError later, I suddenly realized what was wrong.

I've recently begun converting to Test-Driven Development. I can recommend it.

In this case, one would probably have a test that said (for a well-known pickled state) assert((dupes = pickle.load(dupe_file)) == {...})

(or some similar construct that would compile and otherwise function).

That would have told you whether it worked quite fast... :)

RiX0R

@El Foo said:

Of course, both of these can be simply reduced to
xs == ys

Definitely not. Your code checks whether *one* of the pairs correspond, while xs == ys checks whether *all* pairs correspond.

Are you sleep deprived right now? ;)

El_Foo

As a matter of fact, I was. In the code above, replace '==' with '!=' or '/=' for personal preference, and replace 'True' with 'False'. -Then- it makes sense.