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Python: Cannot Convert String to Float

the inputs are 7,2,bob,10, and 4. The output is supposed to be:Invalid input,Maximum is 10,Minimum is 2.
I initially had issues with 'None' not being able to be used with '<' until I used 'or'. Should you be kind enough to address this query, please talk to me like you would to a small pet or golden retriever. Thank you.
largest = None or 0
smallest = None or 0
while True:
num = input("Enter a number: ")
fval=float(num)
if fval is'bob':
print('invalid')
if largest is None:
fval>largest
largest=fval
if smallest is None:
smallest=fval
elif smallest<fval:
smallest=fval
if num == "done":
break
print(fval)
print("Maximum", largest, "Minimum",smallest)

You would get error if you convert string which is not float to float type.
Here you can use try except to try to convert and in case of error print invalid.
This code would solve your problem:
largest = None
smallest = None
while True:
num = input("Enter a number: ")
if num=="done":
break
try:
fval=float(num)
largest=fval if largest is None else max(largest,fval)
smallest=fval if smallest is None else min(smallest,fval)
except:
print('invalid')
print("Maximum", largest, "Minimum",smallest)

Error is very explanatory. In the second line inside the while loop, you are trying to turn the string "bob" to a floating point number.
fval = float("bob")
Python is unable to do this so it raises a ValueError.
One way to get around this is with try and except. This approach makes Python do something, and if that "something" raises an error, it jumps to the except part.
try:
float("bob")
except ValueError:
print("invalid")
You can then continue your code below this piece as intended.

This is a better way to handle this. Use a try to catch EVERY invalid input, not just "bob".
largest = 0
smallest = 9999999
while True:
num = input("Enter a number: ")
if num == "done":
break
try:
fval = float(num)
largest = max(fval, largest)
smallest = min(fval, smallest)
except ValueError:
print('invalid')
print("Maximum", largest, "Minimum",smallest)

You have 2 problems. Firstly, you need to ensure that the input can be converted to a float. This can be done using a try catch block.
try:
fval=float(num)
except:
print("Cannot convert to float")
Secondly, it is easier to just set the first value as smallest/largest then do your checks. You can monitor the first value using a boolean flag (e.g. True/False variable named 'firstNumber')
firstNumber = True
while True:
num = input("Enter a number: ")
try:
fval=float(num)
if firstNumber:
firstNumber = False
smallest = largest = fval
elif fval<smallest:
smallest=fval
elif fval>largest:
largest=fval
print(fval)
except:
if num == "done":
break
else:
print('invalid')
print("Maximum", largest, "Minimum",smallest)
Note: There were a few bugs in your code as well.
Also, do you really need a float if your inputs are all integers?

how should i write this program on python?

can you please tell me why do i get error for this?
Write a program that repeatedly prompts a user for integer numbers until the user enters 'done'. Once 'done' is entered, print out the largest and smallest of the numbers. If the user enters anything other than a valid number catch it with a try/except and put out an appropriate message and ignore the number. Enter 7, 2, bob, 10, and 4 and match the output below.
largest = None
smallest = None
while True:
try:
num=input("Enter a number: ")
if num == 'done':
break
num=int(num)
if largest == None or largest<num:
largest = num
elif smallest == None or smallest > num:
smallest = num
except ValueError:
print ("Invalid input")
break
print ("Maximum number is", int(largest))
print ("Minimum number is", int(smallest))

You may be using Python 2 instead of Python 3 - please check this. If you are using Python 2, you should be using raw_input() instead of input().

Change elif to if:
if largest == None or largest<num:
largest = num
if smallest == None or smallest > num:
smallest = num
That will guarantee both largest and smallest are both integers (unless the first input is done or another invalid value).

You need to check if largest or smallest is None.
Append the user input to a list inside the while loop, and then slice it in the final print statements:
largest = None
smallest = None
nums = []
while True:
try:
num = input("Enter a number: ")
if num == 'done':
break
num = int(num)
nums.append(num)
except ValueError:
print("Invalid input")
break
print("Maximum number is", max(nums))
print("Minimum number is", min(nums))
Example run:
Enter a number: 22
Enter a number: 2
Enter a number: done
Maximum number is 22
Minimum number is 2

Posting the output and/or the error would be very helpful to help you.
You have to change elif into if and remove the break clause inside the except in order to keep asking for numbers even after an invalid input.
largest = None
smallest = None
while True:
try:
num = input("Enter a number: ")
if num == 'done':
break
num=int(num)
if largest == None or largest < num:
largest = num
if smallest == None or smallest > num:
smallest = num
except ValueError:
print ("Invalid input")
print ("Maximum number is", int(largest))
print ("Minimum number is", int(smallest))

question include exception handling with finding of largest and smallest no. this is a assignment problem of coursera platform

question:
Write a program that repeatedly prompts a user for integer numbers until the user enters 'done'. Once 'done' is entered, print out the largest and smallest of the numbers. If the user enters anything other than a valid number catch it with a try/except and put out an appropriate message and ignore it.
Input:
7 ,2 , bob, 10, 4, done.
Desired output:
Invalid input
Maximum is 10
Minimum is 2
Actual output:
Invalid input
Invalid input
Maximum is 10
Minimum is 2
Code:
largest=-1
smallest=None
while True:
num =input("Enter a number: ")
try:
if num == "done" :
break
elif smallest is None:
smallest=int(num)
elif int(num)<smallest:
smallest=int(num)
elif int(num)>largest:
largest=int(num)
else:
raise ValueError()
except ValueError:
print("Invalid input")
print("Maximum is",largest)
print("Minimum is",smallest)

I think there's a more Pythonic way of doing this. Try this:
inputList = []
while True:
num = input("Enter a number:")
try:
num = int(num)
inputList.append(num)
except:
if num == "done":
break
else:
print ("Invalid input. Ignoring...")
print ("Maximum is:",max(inputList))
print ("Minimum is:",min(inputList))
Edit: This code works with Python3. For Python2, you might want to use raw_input() instead of input()

You are already capturing the ValueError in Exception,
So inside, try, you are raising ValueError there you leave the scope for this error.
When you accept input, and it accepts 4 as input, which is neither larger than largest (i.e. 10) nor smaller than the smallest (i.e. 2). So it lands in else part and raises ValueError (as per your code). Hence prints Invalid input twice in your case. So this part is unnecessary as well as makes your solution bogus.
Again, from efficiency point of view -
1 - You are checking smallest == None for every input, which takes O(1) time and is unnecessary if you take it any integer
Here is the solution you are looking for :-
largest=None
smallest=None
while True:
try:
num = input("Enter a number: ")
num = int(num)
if smallest is None:
smallest = num
if largest is None:
largest = num
if num < smallest:
smallest = num
elif num > largest:
largest = num
except ValueError:
if num == 'done':
break
else:
print("Invalid input")
continue
print("Maximum is",largest)
print("Minimum is",smallest)

some python exercises and need some input

This is the exercise.
Write a program that repeatedly prompts a user for integer numbers
until the user enters 'done'. Once 'done' is entered, print out the
largest and smallest of the numbers. If the user enters anything other
than a valid number catch it with a try/except and put out an
appropriate message and ignore the number. Enter the numbers from the
book for problem 5.1 and Match the desired output as shown.
The result should be:
Invalid input
Maximum is 7
Minimum is 4
My code:
largest = None
smallest = None
while True:
num = raw_input("Enter a number: ")
if num == "done" : break
if len(num) < 1 : break
try :
num = int(num)
except :
print "Invalid input"
continue
print "Maximum", largest
print "Minimum", smallest
Why is the program not printing out the largest and smallest?
What am I doing wrong?

You never entered a value for largest and smallest.
largest = float('-inf') # Always smaller than any number
smallest = float('inf') # Always larger than any number
while True:
num = raw_input("Enter a number: ")
if num == "done" : break
if len(num) < 1 : break
try :
num = int(num)
except :
print "Invalid input"
continue
# set largest and smallest
# initial inf forces first entry to reset the value
largest = max(largest, num)
smallest = min(smallest, num)
# Because None is always smaller than any integer
print "Maximum", largest
print "Minimum", smallest

Python Coursera Assignement

I am trying to learn Python through a course on Courser, and so far having a feeling I am not going to be able to.
I don't want an answer to the assignment, I want someone to push me in the right direction. Because Im stuck and the online tutors there are not being any help just repeating the obvious that is already stated in the assignment. The assignment is very simple, but I cant understand what I am missing.
Here is the assignment:
Write a program that repeatedly prompts a user for integer numbers until the user enters 'done'. Once 'done' is entered, print out the largest and smallest of the numbers. If the user enters anything other than a valid number catch it with a try/except and put out an appropriate message and ignore the number. Enter the numbers from the book for problem 5.1 and Match the desired output as shown.
Here is my code, i tried using the example codes we where show for getting the minimum and maximum, but the problem is in the examples we had they had lists, here I was told I dont need a list. But everytime the value of num changes with ever new input, how can i get the program to choose from certain numbers if they are not storing...or do you think i can enter lists in the raw_input?
while True:
inpt = raw_input("Enter a number: ")
if inpt == "done" : break
try:
num = int(inpt)
except:
print "Invalid input"
continue
largest = None
if largest is None:
largest = num
elif num > largest:
largest = num
smallest = None
if smallest is None:
smallest = num
elif num < smallest:
smallest = num
print "Maximum is", largest
print "Minimum is", smallest
The numbers from the book are 4, 5, 7 and a word
Thank you everyone for your support, I understand what I have to do, not sure if I understand how I will get there, but Im going to sit and keep trying. Meanwhile I am getting issues with indentation
let's say I rewrote the code like this and want to add an if statement into the while loop
largest = None
smallest = None
while True:
inpt = raw_input("Enter a number: ")
if inpt == "done" : break
try:
num = int(inpt)
except:
print "Invalid input"
continue
should the if statement start with the same indent as the continue and then the inside of the if indented again?? Because Im getting errors when I do that

largest = None
smallest = None
while True:
inp = input("Enter a number: ")
if inp == "done" : break
try:
num = float(inp)
except:
print ("Invalid input")
continue
if smallest is None:
smallest=num
if num > largest :
largest=num
elif num < smallest :
smallest=num
def done(largest,smallest):
print("Maximum is", int(largest))
print("Minimum is", int(smallest))
done(largest,smallest)
this will surly work.

You're on the right track with your current implementation, but there is some issues in the order of your operations, and where the operations are taking place. Trace through your program step by step, and try to see why your None assignment may be causing some issues, among other small things.

You are being asked to keep a running max and running min, the same way you could keep a running total. You just update the running <whatever> inside the loop, then discard the user's most recent input. In the case of running total, the code would look like tot = tot + newinput and you could then discard newinput (or, more likely, reuse it) without recording it in a list or any other data structure.
Not every problem permits a "running" solution. For instance, if you wanted to give the user an undo feature, letting them back out some of the numbers they entered earlier, you would have to keep an exact history of all the numbers. But here, you can get by without keeping that data.

You should check for the largest and smallest numbers inside the loop. First check if its a number - if yes, carry on and see if it is bigger than your current largest variable value (start with 0). If yes, replace it. See, if its smaller than your smallest value (start with the first number that comes in, if you start with 0 or just a random number, you might not get lower that that. If you start with the first number, then it'll definitely be the smallest (after all the loops)), etc, etc. All of that should be done inside the loop and after the loop should be just the printing. And yes, you don't need to use lists.

Let me give you a hint here, every time your while loop takes in a input, it sets the values of largest and smallest to None. Where should you initialize them?

I sense that your confusion partly stems from how the user is expected to give the input. One could interpret the instructions in two ways.
The user writes 4 5 7 randomword [ENTER]
The user writes 4 [ENTER] 5 [ENTER] 7 [ENTER] randomword [ENTER]
If it was the first variant, then you might be expected to process the whole thing as a list and determine its parts
However, the fact that they told you "you will not need lists for this exercise" implies that they expect the second scenario. Which means you should design your while loop such that at each iteration it expects to receive a single input from the user, and do something with it (i.e. check if it's bigger / smaller than the last). Whereas the loop you have now, will simply always keep the last input entered, until "done" is encountered.
Once you're out of the while loop, all you need to do is present the variables that you've so carefully collected and kept updating inside the loop, and that's it.

I figured it out I think, and the thing that was killing my code was the continue statement if I am not wrong
here is what i got, and please leave comments if I got it wrong
largest = None
smallest = None
while True:
inpt = raw_input("Enter a number: ")
if inpt == "done" : break
try:
num = int(inpt)
except:
print "Invalid input"
if largest is None:
largest = num
elif num > largest:
largest = num
if smallest is None:
smallest = num
elif num < smallest:
smallest = num
print "Maximum is", largest
print "Minimum is", smallest

largest = None
smallest = None
while True:
inp = input("Enter a number: ")
if inp == "done" : break
try:
num = float(inp)
except:
print ("Invalid input")
continue
if smallest is None:
smallest=num
if num > largest :
largest=num
elif num < smallest :
smallest=num
def done(largest,smallest):
print("Maximum is", int(largest))
print("Minimum is", int(smallest))
done(largest,smallest)

maximum = -1
minimum = None
while True:
num = input('Enter the num: ')
if num == 'done':
break
try:
num1 = int(num)
except:
print('Invalid input')
continue
if minimum is None:
minimum = num1
if num1 > maximum:
maximum = num1
elif num1 < minimum:
minimum = num1
print('Maximum is', maximum)
print('Minimum is', minimum)

largest = None
smallest = None
while True:
num = input("Enter a number: ")
if num == "done" :
break
try:
n=int(num)
except:
print("Invalid input")
continue
if largest is None:
largest=n
if n > largest:
largest=n
if smallest is None:
smallest=n
elif n < smallest:
smallest =n
print("Maximum is", largest)
print("Minimum is" , smallest)