int vs. float

[adam14711993 at gmail.com]

Hello,

My computer programming professor challenged me to figure out a way to manipulate my program to display one error message if the user input is a zero or a negative number, and a separate error message if the user input is a decimal number.  My program starts out:

number_purchases_str = int(input("Enter the number of packages you're buying:"))
number_purchases = int(number_purchases_str)
format(number_purchases, "12,")

So if the user input is, for example, -9 or 0, the program will result with: Sorry, you must order at least one package."

What I cannot figure out is how to write it so that if my user input is, for example, 1.5, the program will result with: Sorry, you can only order whole packages.

I understand that because I am starting out by assigning my number_purchases_str to be an int, when the user enters a float that is a conflict and will crash.

My professor apparently believes there is a way to accomplish this.  Any help or advice would be greatly appreciated.

Thank you.