Calculator App

Architectural Plan

To keep the project maintainable and scalable, the application is divided into three layers:

1. UI Layer (Flet)

Responsible for:

• User interactions
• Display updates
• Event handling

2. Logic Layer

Responsible for:

• Calculator state management
• Tracking operands and operators
• Processing user input

3. Utility Layer

Responsible for:

• Mathematical operations
• Precision handling
• Error handling
• Secure calculations without using eval()

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Development Roadmap

Phase 1 — State Management

Goal: Build the calculator engine.

• Create CalculatorEngine
• Store current operand
• Store previous operand
• Store selected operator
• Handle input flow before pressing "="

Phase 2 — Refined UI

Goal: Improve the user experience.

• Replace standard buttons with custom Container widgets
• Create a minimalist layout
• Improve spacing and alignment
• Add consistent styling

Phase 3 — Robust Parsing

Goal: Make calculations reliable.

• Implement calculate()
• Handle division by zero
• Handle invalid input
• Improve floating-point precision

Phase 4 — History Logging

Goal: Track previous calculations.

• Add history panel
• Create scrollable ListView
• Save operations locally
• Load history on startup

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Project Structure

calculator_app/
├── main.py           # Flet entry point
├── engine.py         # Calculator logic
├── styles.py         # Colors, fonts, themes
└── history.json      # Optional persistent storage

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Technical Strategy: Avoiding eval()

Instead of evaluating raw strings, operators are mapped directly to Python functions.

import operator

ops = {
    "+": operator.add,
    "-": operator.sub,
    "*": operator.mul,
    "/": operator.truediv,
}

# Example
# result = ops[operator_symbol](float(num1), float(num2))

Benefits

• Safer than eval()
• Easier to debug
• Easier to extend
• Better control over errors

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Implementation Notes

UI Direction

• Material Design 3 inspired
• Dark theme
• High contrast colors
• Minimalist layout

State Flow

INPUTTING_FIRST_NUMBER
        ↓
WAITING_FOR_OPERATOR
        ↓
INPUTTING_SECOND_NUMBER
        ↓
CALCULATE_RESULT

Input Validation

Use regex validation to ensure user input is valid before passing values to the calculation engine.

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Current Status

Feature	Status
Project Structure	⬜ Planned
Calculator Engine	⬜ Planned
UI Components	⬜ Planned
Parsing & Validation	⬜ Planned
History Logging	⬜ Planned
Testing	⬜ Planned