Python hourglass graph drawing: How to avoid variable undefined errors?

Getting started with Python: Draw hourglass graphics and perform input verification

This article will explain how to fix undefined variable errors encountered by a Python novice when writing an hourglass graph drawing program. The goal of the program is to draw an hourglass pattern composed of sym characters based on the integer n and character sym entered by the user. There is a NameError error in the original code, which prompts that sym variable is not defined. Let's analyze and solve this problem.

The original code mainly contains two functions: is_integer_string is used to verify whether the input is an integer, print_hourglass is used to draw the hourglass pattern. However, problems occur when processing user input, resulting in sym variable undefined when the print_hourglass function is called.

The original code tries to read both n and sym using map(int, input().split()) . This assumes that the user enters two integers separated by spaces. But in reality, sym should be a character or string, not an integer. Therefore, map(int, input().split()) throws ValueError exception and sym is not assigned correctly.

The improved code is as follows:

 def is_integer_string(s):
    if not s.isdigit() and (s[0] != '-' or not s[1:].isdigit()):
        return False
    return True

def print_hourglass(n, sym):
    i = 1
    while n >= i * i:
        print(' ' * (i - 1) sym * (i * 2 -1)) # Modify the formula to make the hourglass more symmetrical print(' ' * (i - 1) sym * ((i 1) * 2 -1)) # Modify the formula to make the hourglass more symmetrical i = 2
    while i > 0:
        print(' ' * (i - 1) sym * (i * 2 -1)) # Modify the formula to make the hourglass more symmetrical i -= 2

try:
    n_input = input("Please enter the integer n:")
    if is_integer_string(n_input):
        n = int(n_input)
        sym = input("Please enter the character sym:")
        print_hourglass(n, sym)
    else:
        print("The input n is not an integer")
except ValueError:
    print("Input error, please re-enter")
except Exception as e:
    print(f"Error occurred: {e}")

The improved code first corrected the is_integer_string function so that it can handle negative numbers correctly. More importantly, it processes the inputs of n and sym separately. First read n , verify whether it is an integer, and then read sym to avoid errors caused by map function. In this way, the sym variable gets the correct assignment before print_hourglass function call, thus solving the NameError error. The code also makes more robust error handling of inputs and adds more user-friendly prompts. At the same time, the formula for calculating the number of characters in print_hourglass function was also corrected to make the hourglass graph more symmetric. Finally, a more general except block was added to catch other possible exceptions.

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