On 11/02/2014 03:05 PM, Clayton Kirkwood wrote:
-----Original Message-----
From: Tutor [mailto:tutor-bounces+crk=godblessthe...@python.org] On
Behalf Of Alan Gauld
Sent: Sunday, November 02, 2014 1:04 AM
To: tutor@python.org
Subject: Re: [Tutor] all right students, what do we learn
On 02/11/14 05:43, Clayton Kirkwood wrote:
So I head down the path of all sorts of variations of the following.
And I mean all sorts. Instead of [reenter, key_columns] I used a
single list. Instead of the exec('reenter = True') I used a simple
reenter=True (and an eval and compile which I don't totally understand
yet).
Don't use exec, eval or compile. Consider them for experts only and
hardly ever needed.
They are extremely dangerous and doubly so if you don't know exactly
what you are doing. You could, for example, wind up accidentally
deleting your files or formatting your hard disk.
Got it. I'll wait until next month:<}}}
out that python doesn't seem to like an assignment in the overall if
clause - error.
Correct, the if statement requires an expression (specifically a boolean
expression). And assignments are not expressions.
My thinking was that I was making a Boolean value:
]=[(reenter, key) for key in key_list if
((key in key_list0)) or (print("Error:", key, "not available, start
again") and exec('reenter = True', ))]
In the first case, key is in key_list0 so that is a True and we are finished
for that key. In the other case, key is not in key_list0 so we go on past
the or. I print which returns a non-False, ergo a True,
But it doesn't. print() returns None, which is false.
and then I make an
assignment which should be a True. This was I am able to set reenter to
True. Assignments create a Boolean True (or should).
Assignment is a type of statement. There's nothing created. The lhs is
bound to the rhs. And the "=" is NOT an operator, and may not be used
in an expression. Python is not C.
If you're trying to hack an expression with side effect, perhaps you
should write a function:
def force():
global reenter
reenter = True
return True
The other issue is that I had the outside reenter set as true to start the
for and turn it False so that the for won't re-occur unless I have a bad key
and force reenter to be a True.
Weird thing, entering a y would make the final
print spit out (False, 'a') (False, 'o') but the entry of hhh
caused
the final print to spit out []
Why do you think that weird? Its exactly what I'd expect. You don't have
a key 'hhh' in the data you showed us.
Remember that the comprehension is trying to build a list containing
only those output expressions that match the if clause. If the if fails
then nothing gets put in the output.
Again, the code, if the equal sign worked would always force a True. In one
case reenter doesn't change, in the second case, reenter does change but the
expression is still True.
It could be, if you rechecked your assumptions, such as the return value
of print()
Which I don't quite understand. The reason is because I set up the if
statement to be True either way. With an a it would go thru and
create the proper output.
Can you explain in English what you want the output to be?
The reason I ask is that you seem to be creating a list of pairs where
the first element is the result of the 'in' test and the second is the
value. Do you really want that or do you just want the ones that match
either true or false?
What I would expect from [('a', 'apple'), ('b', 'banana')] and entering a y
Would be:
[('a',True), ('y', False)]
But "a" isn't in the list [('a', 'apple'), ('b', 'banana')]. Maybe
you'd better get the basics right before you try to get obfuscated with
your code. Write real data for a real function that contains a real
loop that produces what you really want. Then you can start with the
obfuscation.
--
DaveA
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