Simple MPG calculator in Python

I don't think try except else is a standard python construction - does it even execute? In that sense this does not look 'well formed'. Does this execute? It does not work when copy pasted but I think that's just formatting issues.

It's hard to write this without some try's, since the standard string > float conversion method is

 dist = input("enter distance")
 dist_num = float(dist)

which will except if dist is not a number; most python coders just try/catch around it. Without try's you'd have to provide a way of checking for floats that doesn't require a try/catch; maybe it's the point of the assignment? Apart from input validation, it's just doing a division otherwise.

To get a float from a string without a try, you could do:

def is_number(input_string):
    for char in input_string: 
        if char not in '0123456789.': return False
    return True

before the string is converted to a float (this will also filter out negative numbers for free as written)

In terms of style and construction, the conditions for the mileage and consumption are the same: positive numbers > 0. The only differences are the prompts and error messages. So it makes sense to write one function and pass the prompts in to it twice:

def get_input(prompt, error_msg ):
    value = input(prompt)
    if not is_number(value):
       print error_msg
       return get_input(prompt, error_msg)

    num = float(value)
    if num <= 0:
       print error_msg
       return get_input(prompt, error_msg)

    return num

This will just keep asking until the user gives a positive number. The rest is easy:

 miles = get_input("Enter distance traveled(in miles):", "Distance traveled must be a number larger than 0")
 gas = get_input("Enter gasoline consumed (in gallons):", "Gasoline consumed must be a number larger than 0")

 mpg = miles/gas

This could actually be improved further by replacing 'is_number' with a function that did the float conversion and converted all non-numeric values to -1; then you could just have a single test in the get_input function. I did it 'the long way' so the logic is clearer.

The last thing to look at is the layout, it would typically look like

def is_number():
    ..... etc

def get_input():
    ..... etc

def main():
    miles = get_input("Enter distance traveled(in miles):", "Distance traveled must be a number larger than 0")
   gas = get_input("Enter gasoline consumed (in gallons):", "Gasoline consumed must be a number larger than 0")
    ....... etc

That would make it easier to reuse the pieces. The standard pythonic way to call a main function is

if __name__ == 'main':
   main()

This makes sure the code only fires when the file is executed as a script, not when it's is imported as a module.

I have three main remarks:

• Recursion is not really an appropriate retry mechanism.

  Try this: hold down Enter for a dozen responses, then hit ControlC. As you see, the stack trace is quite deep. It would be better for the call stack to be the same regardless of how the user arrived at a certain point in the code, because invariants make software easier to test and debug. You're using recursion as a GOTO, except it's worse because it leaves a trace. Instead, the validation should be done in a loop, which you break out of when valid input is obtained.

• The code structure is a bit repetitive.

  The routines to input the distance and gas consumption are actually quite similar, and could be written using shared code.

• Trial division is a poor way to test for a zero value.

  You test whether dist is zero with

  # if dist is 0.0, print error and restart
  try:
      x = 1 / dist
  except ZeroDivisionError:
      print('Miles traveled cannot be 0!')
      main()
  

  There are more straightforward ways to test for a zero value! You end up discarding x, so the trial division isn't even useful work.

  On further thought, your validation is not quite right. If you're going to validate your input, you might as well prohibit negative numbers. It is possible for the distance to be zero, when the car has been idling. (The zero-distance case would only be problematic when displaying efficiency in "metric" units of L / 100km.)

In conclusion, your use of exceptions for validation is partly inappropriate (catching ZeroDivisionError to test for zero, especially for dist, from a logical viewpoint as well as a code expression viewpoint) and partly appropriate (catching ValueError to test for strings that cannot be interpreted as numbers). I don't see any alternative to catching ValueError, except perhaps checking the input string against a regular expression \d+(?:\.(?:\d*))?, and I would consider ValueError to be the superior approach. I think it would be fair to challenge your professor to propose a better alternative — in my opinion, the ball is in her court.

A minor note: your comments are inconsistent with your code. Your code has a variable named gcon; the comments mention guse.

Suggested solution:

def ask(prompt, typeconv, typeconv_err, validation=None, validation_err=None):
    """
    Displays the prompt and asks for user input.  The result is passed through
    the typeconv function, then possibly a validation function as well.  If 
    either the type conversion fails (with a ValueError) or the validation 
    fails (by being false value), then the respective error message is 
    displayed, and the prompt is displayed anew.
    """
    while True:
        val = input(prompt)
        try:
            val = typeconf(val)
        except ValueError:
            print(typeconv_err % (val))
        else:
            if validation is not None and not validation(val):
                print(validation_err % (val))
            else:
                return val

def main():
    dist = ask('Enter distance traveled(in miles): ',
               float,
               'Distance traveled must be a number! You did not travel "%s" miles!',
               lambda dist: dist >= 0.0,
               'Miles traveled cannot be negative!')

    gcon = ask('Enter gas consumed traveling(in gallons): ',
               float,
               'Gas consumed must be a number. You did not use "%s" gallons of gas!',
               lambda gcon: gcon > 0.0,
               'Gas consumed cannot be 0 or negative!')

    # print mpg
    mpg = dist / gcon
    print('Average miles per gallon(mpg):', format(mpg, '5.2f'))

    # print request for enter keypress to end
    input('press enter to continue')

# Call main function
main()