How to solve the NameError error when printing hourglass patterns in Python?

Python beginner encounters NameError when printing hourglass patterns: Problem analysis and code improvement

This article provides a detailed solution to NameError: name 'sym' is not defined error encountered by Python beginners when printing hourglass patterns. The original code intends to use the print_hourglass function to print an hourglass pattern composed of the user-specified characters, but the error is caused by improper input processing.

The root of the problem lies in the input processing of the original code: it tries to read both the line number and the characters, but if the user input format is incorrect (for example, just enter one number or enter a non-numeric character), map(int, input().split()) function raises ValueError exception. More importantly, even if there is no ValueError , the sym variable will be undefined if the user does not provide a second input (character), resulting in NameError .

The improved code solves this problem with more robust input processing and variable definitions:

 def is_integer_string(s):
    """Check whether the input string is an integer"""
    try:
        int(s)
        return True
    except ValueError:
        return False

def print_hourglass(n, sym):
    """Print hourglass pattern"""
    i = 1
    while n >= i * i:
        print(' ' * (i - 1) sym * (i * 2 -1)) # Correction formula print(' ' * (i - 1) sym * ((i 1) * 2 -1)) # Correction formula i = 2
    while i > 0:
        print(' ' * (i - 1) sym * (i * 2 -1)) # Modify formula i -= 2

try:
    n_str = input("Please enter the number of lines of the hourglass (odd):")
    if is_integer_string(n_str):
        n = int(n_str)
        if n % 2 == 0:
            print("Please enter odd numbers")
        else:
            sym = input("Please enter the character that forms the hourglass:")
            print_hourglass(n, sym)
    else:
        print("Invalid input, please enter integer")
except Exception as e:
    print(f"Error occurred: {e}")

Improvements:

1. Clearer input prompts : The code adds input prompts to guide the user to enter the number of lines and characters.
2. Read input separately : The code reads the number of lines and characters separately, avoiding potential problems with map function.
3. Integer Check : The is_integer_string function simplifies, using try-except block to check whether the input is an integer more concisely.
4. Odd number check : Added a check on whether the number of input lines is odd to ensure the correct display of the hourglass pattern.
5. Error handling : Use try-except block to catch all possible exceptions and print friendly error messages.
6. Formula correction : The formula for printing the hourglass pattern in the original code has a slight flaw and has been corrected to enable it to print the hourglass pattern correctly.

With these improvements, the code is more robust, able to effectively handle various user inputs, avoid NameError errors, and print out the correct hourglass pattern.

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