Input and Output

Console Input

The console is a text interface for input to and output from the computer. In Spyder, the iPython window itself can serve as a console. In JupyterLab the output console is indicated by lines marked with Out [] whereas a reference to an input console will open a textbox.

To read input from the console we use the input() function (Python 3+). In Python 2.7 the equivalent is raw_input(). Any string within the parentheses is optional and, if present, it will be printed to prompt the user. The input from the user is captured as a string and returned; it must be stored for any subsequent use.

The input function returns only a string. If you need to use the values as any other type, you must perform the conversion yourself.


weight=input("Enter your weight in pounds:")
weight=float(input("Enter your weight in pounds:"))

Console Output

We have already been using the print function. Let us now examine it in more detail.

The print function

  • always inserts a space between its arguments
  • always adds a newline character unless the end argument is added.
    • print(var,end="")

Messages can be added in between variables.

print("Your BMI is",w/ht**2)


Use Spyder or another IDE to write a complete program to compute BMI from weight and height input from a user. First request the user’s choice of units. We have not spent much time with strings yet so you may use a digit to indicate the user’s choice, but remember it will still be a string on input. Then request weight and height. You will need to convert these from strings. Look up the correct conversion factors for Imperial to metric units. Compute the BMI.

The categories are

BMI Category
less than 18.5 Underweight
18.5 to 25.0 Normal
25.0 to 30.0 Overweight
30.0 to 35.0 Obese Class I
35.0 to 40.0 Obese Class II
more than 40.0 Obese Class III

Print the user’s BMI value and category.

Example solution

unit=int(float(input("Enter 1 for Imperial or 2 for metric units:")))

if unit==1:
    weight=float(input("Enter your weight in pounds:"))
    height=float(input("Enter your height in inches:"))
elif unit==2:
    weight=float(input("Enter your weight in kg:"))
    height=float(input("Enter your height in m:"))
    print("Invalid unit request")

if         BMI < 18.5 : category="Underweight"
if 18.5 <= BMI < 25.0 : category="Normal"
if 25.0 <= BMI < 30.0 : category="Overweight"
if 30.0 <= BMI < 35.0 : category="Obese Class I"
if 35.0 <= BMI < 40.0 : category="Obese Class II"
if 40.0 <= BMI < 45.0 : category="Obese Class III"
if         BMI>= 45.0 : category="Obese Class IV"

print("Your BMI is {:.2f} and you are {}".format(BMI,category))