Python 2.7 try and except ValueError

Question

I query user input which is expected to be an int by using int(raw_input(...))

However when the user doesn't enter an integer, i.e. just hits return, I get a ValueError.

def inputValue(inputMatrix, rangeRows, rangeCols, defaultValue, playerValue):
    rowPos = int(raw_input("Please enter the row, 0 indexed."))
    colPos = int(raw_input("Please enter the column, 0 indexed."))
    while True:
        #Test if valid row col position and position does not have default value
        if rangeRows.count(rowPos) == 1 and rangeCols.count(colPos) == 1 and inputMatrix[rowPos][colPos] == defaultValue:
            inputMatrix[rowPos][colPos] = playerValue
            break
        else:
            print "Either the RowCol Position doesn't exist or it is already filled in."
            rowPos = int(raw_input("Please enter the row, 0 indexed."))
            colPos = int(raw_input("Please enter the column, 0 indexed."))
    return inputMatrix

I tried to be smart and use try and except to catch the ValueError, print a warning to the user and then call the inputValue() again. Then it works when the user enters return to the query but falls over when the user correctly then enters an integer

Below is the part of the amended code with the try and except:

def inputValue(inputMatrix, rangeRows, rangeCols, defaultValue, playerValue):
    try:
        rowPos = int(raw_input("Please enter the row, 0 indexed."))
    except ValueError:
        print "Please enter a valid input."
        inputValue(inputMatrix, rangeRows, rangeCols, defaultValue, playerValue)   
    try:
        colPos = int(raw_input("Please enter the column, 0 indexed."))
    except ValueError:
        print "Please enter a valid input."
        inputValue(inputMatrix, rangeRows, rangeCols, defaultValue, playerValue)

Is there a question in there somewhere?

Marcelo Cantos
– Marcelo Cantos

2011-02-17 05:29:13 +00:00
Commented Feb 17, 2011 at 5:29 — Marcelo Cantos
– Marcelo Cantos, Commented Feb 17, 2011 at 5:29

mtrw · Accepted Answer · 2011-02-17 06:52:48Z

37

A quick and dirty solution is:

parsed = False
while not parsed:
    try:
        x = int(raw_input('Enter the value:'))
        parsed = True # we only get here if the previous line didn't throw an exception
    except ValueError:
        print 'Invalid value!'

This will keep prompting the user for input until parsed is True which will only happen if there was no exception.

edited Feb 17, 2011 at 6:52

answered Feb 17, 2011 at 6:47

mtrw

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Comments

Alexander Lebedev · Accepted Answer · 2011-02-17 05:53:22Z

3

Instead of calling inputValue recursively, you need to replace raw_input with your own function with validation and retry. Something like this:

def user_int(msg):
  try:
    return int(raw_input(msg))
  except ValueError:
    return user_int("Entered value is invalid, please try again")

answered Feb 17, 2011 at 5:53

Alexander Lebedev

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Comments

Jesse Aldridge · Accepted Answer · 2011-02-17 05:50:03Z

Is something like this what you're going for?

def inputValue(inputMatrix, defaultValue, playerValue):
    while True:
        try:
            rowPos = int(raw_input("Please enter the row, 0 indexed."))
            colPos = int(raw_input("Please enter the column, 0 indexed."))
        except ValueError:
            continue
        if inputMatrix[rowPos][colPos] == defaultValue:
            inputMatrix[rowPos][colPos] = playerValue
            break
    return inputMatrix

print inputValue([[0,0,0], [0,0,0], [0,0,0]], 0, 1)

You were right to try and handle the exception, but you don't seem to understand how functions work... Calling inputValue from within inputValue is called recursion, and it's probably not what you want here.

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