from tkinter import *

# Trial - Entry Validation v1
# Purpose: Learn the basic approach to checking whether a user has entered
#          a valid number in an Entry widget using try/except

class ValidationTrial:

    def __init__(self):

        self.frame = Frame(padx=10, pady=10)
        self.frame.grid()

        self.heading = Label(self.frame,
                             text="Entry Validation Trial - v1",
                             font=("Arial", "16", "bold"))
        self.heading.grid(row=0)

        instructions = ("Enter different values and press Check. "
                        "Try: a number, text, a negative number, "
                        "a decimal, nothing.")
        Label(self.frame,
              text=instructions,
              wraplength=300,
              font=("Arial", "11"),
              fg="#666666").grid(row=1, pady=5)

        self.entry = Entry(self.frame, font=("Arial", "14"), width=20)
        self.entry.grid(row=2, padx=10, pady=10)

        self.check_button = Button(self.frame,
                                   text="Check Input",
                                   font=("Arial", "12", "bold"),
                                   bg="#009900", fg="#FFFFFF",
                                   width=12,
                                   command=self.check_input)
        self.check_button.grid(row=3, pady=5)

        self.result_label = Label(self.frame,
                                  text="",
                                  font=("Arial", "12"))
        self.result_label.grid(row=4, pady=10)

    def check_input(self):
        """
        Validates the entry box content.
        Steps:
        1. Read the raw string with .get()
        2. Try to convert to float
        3. If it fails, ValueError is raised - catch it
        4. If it succeeds, check it's >= 0
        """

        raw = self.entry.get()

        # Reset styling
        self.entry.config(bg="#FFFFFF")

        try:
            value = float(raw)

            if value >= 0:
                # Valid!
                self.result_label.config(
                    text=f"Valid number: {value}",
                    fg="#009900"
                )
            else:
                # Number but negative
                self.result_label.config(
                    text="Error: number must be 0 or greater",
                    fg="#9C0000"
                )
                self.entry.config(bg="#F4CCCC")
                self.entry.delete(0, END)

        except ValueError:
            # Not a number at all
            self.result_label.config(
                text="Error: please enter a number",
                fg="#9C0000"
            )
            self.entry.config(bg="#F4CCCC")
            self.entry.delete(0, END)


# main routine
if __name__ == "__main__":
    root = Tk()
    root.title("Validation Trial v1")
    ValidationTrial()
    root.mainloop()