Showing Revision 1 created 04/23/2012 by Amara Bot.

1. Title:
   02-47 Translation Tables
2. Description:

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   Now I'm going to show you how these translation tables work.
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   I'm going to define a variable called "table"
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   that's using the string.maketrans function, which makes a translation table,
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   and I'm going to tranlate from the characters 'ABC' to '123.'
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   I can give any number of characters here--
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   the characters I want to replace and the ones I want to replace them with.
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   I should say that this is using the string module,
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    so somewhere we have to say import string before we start doing any of this.
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    You only have to do that import once, of course.
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    Now I'm going to define a formula f to be a simple formula A plus B equals C.
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    Then I'm going to call the translate method of the formula f and pass it this translation table.
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    That will evaluate to the string 1 plus 2 equals 3.
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    It has taken each of the elements in the table,
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    and they correspond A to 1, B to 2, C to 3,
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    substituted those into f and given me back a brand new string.
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    Now if I go ahead and evaluate f.translate of table,
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    which is 1 plus 2 equals 3, then that will give me the result True,
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    because 1 plus 2 is 3, and that's a legal Python expression.
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    Now what I want you to do is to define for me the function "valid,"
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    which takes a filled-in formula like 1 plus 2 equals 3, filled-in formula f,
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    and returns True or False. True if the formula is, in fact, valid.
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    If it represents a true equation like this.
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    And False if it represents an invalid equation like 1 plus 3 equals 3.
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    Or it should also return False if it represents a error like 1 divided by 0 equals 3.
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    That wouldn't return True or False, that would signal an error,
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    and I want you to handle that within the code for valid.
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    I'll give you a hint,
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    which is you should consider using a try statement.
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    Try, do something, and then you can say "except ZeroDivisionError" something.
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    What that does is it executes the main body in which you can test if evaluating
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    this expression f is true or not and return appropriately,
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    but if evaluating the expression f causes a zero division error,
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    then this clause will catch it, and then you can do the appropriate thing here.
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    You should also think about if there's anything else that can go wrong
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    within the execution of valid.
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    Here's my version of the solution.
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    I'm defining valid, takes filled-in formula f, and it's going to return True.
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    The main part is if we evaluate f and if that's true, then we should return True,
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    but I also had to check for the zero division error
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    and even to be a little bit more sore here,
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    I ended up checking for arithmetic error, which is a super class of zero division error.
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    It covers a few additional things like overflow of really big numbers.
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    You didn't have to do that. I would've been fine to just catch the zero division error.
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    If there is such an error, then you should return False.
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    But I did one more thing here, and it looks kind of complicated.
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    I'm using a regular expression search, and let's look at exactly what's going on
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    in this confusing part of the clause here.