如何写一个输入，要求用户使用字母和数字键入？

Question

I am writing a line where I ask the user to input their school ID, which requires both letters and numbers. How can I do this? When I input a number as my answer when asked, my program works. If I type a letter with the number I get the error. Here is my code thus far using Python 3.6 IDLE: def main():

#Introduction
print ("Welcome to my program!\n")

#Variables with starting values waiting for user input

studentid = ""
gpa = 0.0
classnumber = ""

#Prompt for studentID
studentid = eval(input("Please enter your student ID: "))
#Prompt for GPA
gpa = eval(input("Please enter your GPA: "))
#Prompt for classnumber
classnumber = eval(input("Please enter your class information:"))

#display the users information
#StudentID: C100556789
#GPA: 4.0
#Class: COP1000

print ("\nStudent ID:\t", studentid)
print ("GPA:\t", gpa)
print ("Class information:\t", classnumber)

And here is my error:

 Type "copyright", "credits" or "license()" for more information.
 >>>   RESTART: C:\Users\Rene Delgado\Documents\School\Fall 2018\School Code\Student Program.py 
 >>> main() Welcome to my program!

 Please enter your student ID: C100556789 Traceback (most recent call
 last):   File "<pyshell#0>", line 1, in <module>
     main()   File "C:\Users\Rene Delgado\Documents\School\Fall 2018\School Code\Student Program.py", line 18, in main
     studentid = eval(input("Please enter your student ID: "))   File "<string>", line 1, in <module> NameError: name 'C100556789' is not
 defined

Answer 1

input returns a string that the user inputs, which is what you want already. You should not use eval to evaluate the string as a Python expression, which would generate an error for an input with letters because Python would interpret it as a variable name that does not exist. Simply remove calls to eval and your code would work:

#Prompt for studentID
studentid = input("Please enter your student ID: ")
#Prompt for GPA
gpa = input("Please enter your GPA: ")
#Prompt for classnumber
classnumber = input("Please enter your class information:")

Note that if you want to treat gpa as a number for numeric operations, you should also convert the input to a floating number with float() :

gpa = float(input("Please enter your GPA: "))