Noob Question: Force input to be int?

[wd.jonsson]

Hello everyone!
I have a piece of code that looks like this:

if len(BuildList) > 0:
print "The script found %d game directories:" % len(BuildList)
print
num = 0
for i in BuildList:
print str(num) +" " + i
num = num + 1
print
print "Select a build number from 0 to " + str(len(BuildList) - 1)
buildNum = int(raw_input('Select build #> '))

while buildNum > (len(BuildList) -1) or buildNum <= -1:
print
print "Error: Invalid build number!"
print "Select a build number from 0 to " + str(len(BuildList) -
1)
print
buildNum = int(raw_input('Select build: '))

The problem is with the while buildNum-loop. If the user enters a
non-numeric value in the buildNum input, the scripts throws an
exception. I need to constrict the user to ONLY use integers in the
input box. How can I solve this issue?

 

[Daniel Nogradi]

I have a piece of code that looks like this:

if len(BuildList) > 0:
print "The script found %d game directories:" % len(BuildList)
print
num = 0
for i in BuildList:
print str(num) +" " + i
num = num + 1
print
print "Select a build number from 0 to " + str(len(BuildList) - 1)
buildNum = int(raw_input('Select build #> '))

while buildNum > (len(BuildList) -1) or buildNum <= -1:
print
print "Error: Invalid build number!"
print "Select a build number from 0 to " + str(len(BuildList) -
1)
print
buildNum = int(raw_input('Select build: '))

The problem is with the while buildNum-loop. If the user enters a
non-numeric value in the buildNum input, the scripts throws an
exception. I need to constrict the user to ONLY use integers in the
input box. How can I solve this issue?

How about this:

while True:
try:
x = int( raw_input( 'Enter an integer: ' ) )
break
except:
pass

print 'Okay, now I have this integer: %d' % x


HTH,
Daniel

 

[Paul Rubin]

print "Select a build number from 0 to " + str(len(BuildList) - 1)
buildNum = int(raw_input('Select build #> '))

while buildNum > (len(BuildList) -1) or buildNum <= -1:
print
print "Error: Invalid build number!"
print "Select a build number from 0 to " + str(len(BuildList) - 1)
print
buildNum = int(raw_input('Select build: '))

The problem is with the while buildNum-loop. If the user enters a
non-numeric value in the buildNum input, the scripts throws an
exception. I need to constrict the user to ONLY use integers in the
input box. How can I solve this issue?

You can either validate the input string before trying to convert
it ("look before you leap") or attempt conversion and handle the
exception if you get one ("it's easier to ask forgiveness than
permission"). The second pattern is generally preferable since it
means you don't duplicate input validation logic all over your
program.

The below is untested but is a partial rewrite of your code above,
intended to illustrate a few normal Python practices (I hope I haven't
introduced bugs) besides the input checking:

# just loop til you get a valid number, reading the number at the
# top of the loop. No need to read it in multiple places.
while True:
# you don't have to convert int to str for use in a print statement.
# the print statement takes care of the conversion automatically.
print "Select a build number from 0 to", len(BuildList) - 1
buildNumStr = raw_input('Select build #> ')

# attempt conversion to int, which might fail; and catch the
# exception if it fails
try:
buildNum = int(buildNumStr)
except ValueError:
buildNum = -1 # just treat this as an invalid value

# you can say a < x < b instead of (a < x) and (x < b)
if 0 <= buildNum < len(BuildList):
break # you've got a valid number, so exit the loop

# print error message and go back to top of loop
print
print "Error: Invalid build number!"

 

[Daniel Jonsson]

Ah, thank you for the respone!
I have not gotten around to test it yet, but I hope it will work!
-Daniel

2007-01-23 10:59:37
(e-mail address removed) wrote in message

 

[consmash]

Hello everyone!
I have a piece of code that looks like this:

if len(BuildList) > 0:
print "The script found %d game directories:" % len(BuildList)
print
num = 0
for i in BuildList:
print str(num) +" " + i
num = num + 1
print
print "Select a build number from 0 to " + str(len(BuildList) - 1)
buildNum = int(raw_input('Select build #> '))

while buildNum > (len(BuildList) -1) or buildNum <= -1:
print
print "Error: Invalid build number!"
print "Select a build number from 0 to " + str(len(BuildList) -
1)
print
buildNum = int(raw_input('Select build: '))

The problem is with the while buildNum-loop. If the user enters a
non-numeric value in the buildNum input, the scripts throws an
exception. I need to constrict the user to ONLY use integers in the
input box. How can I solve this issue?

Also if you would like to try mentioned approach 'Look before you
leap', this code should be quite clear:

buildNumber = ('foo', 'bar')
n = len(buildNumber)
loop = True
while loop:
input = raw_input('select a build number from 0 to %d: ' % (n-1))
if input.isdigit():
ch = int(input)
loop = not ch in range(n) # if ch is NOT in range of [0, n),
then it is ok to loop
if loop:
print 'Error! Invalid build number'
print 'Choice: %d' % ch

In fact I don't know if it applies to python, but in conventional
languages it is a bad habit to use exceptions for every simple test as
it takes time to jump around code. So, correct me please.