ploughshares/check_contrast.py

#!/usr/bin/env python3
import re
import math
import os
import sys

def luminance(r, g, b):
    """
    Calculate the relative luminance of an RGB color
    Formula from WCAG 2.0: https://www.w3.org/TR/WCAG20-TECHS/G17.html
    """
    r = r / 255
    g = g / 255
    b = b / 255
    
    r = r / 12.92 if r <= 0.03928 else ((r + 0.055) / 1.055) ** 2.4
    g = g / 12.92 if g <= 0.03928 else ((g + 0.055) / 1.055) ** 2.4
    b = b / 12.92 if b <= 0.03928 else ((b + 0.055) / 1.055) ** 2.4
    
    return 0.2126 * r + 0.7152 * g + 0.0722 * b

def contrast_ratio(lum1, lum2):
    """
    Calculate contrast ratio between two luminance values
    Formula from WCAG 2.0: https://www.w3.org/TR/WCAG20-TECHS/G17.html
    """
    lighter = max(lum1, lum2)
    darker = min(lum1, lum2)
    return (lighter + 0.05) / (darker + 0.05)

def hex_to_rgb(hex_color):
    """Convert hex color to RGB tuple"""
    hex_color = hex_color.lstrip('#')
    if len(hex_color) == 3:
        hex_color = ''.join([c*2 for c in hex_color])
    return tuple(int(hex_color[i:i+2], 16) for i in (0, 2, 4))

def parse_css_colors(css_file):
    """Extract color definitions from CSS file"""
    with open(css_file, 'r') as f:
        css_content = f.read()
    
    # Find root variables
    root_vars = {}
    root_match = re.search(r':root\s*{([^}]+)}', css_content)
    if root_match:
        root_content = root_match.group(1)
        var_matches = re.findall(r'--([a-zA-Z0-9-]+):\s*([^;]+);', root_content)
        for name, value in var_matches:
            if '#' in value:
                root_vars[f'--{name}'] = value.strip()
    
    # Find color properties
    color_pairs = []
    
    # Background-color and color pairs
    elements = re.findall(r'([.#]?[a-zA-Z0-9_-]+)\s*{([^}]+)}', css_content)
    for selector, properties in elements:
        bg_color = None
        text_color = None
        
        bg_match = re.search(r'background-color:\s*([^;]+);', properties)
        if bg_match:
            bg_color = bg_match.group(1).strip()
            
        color_match = re.search(r'(?<!background-)color:\s*([^;]+);', properties)
        if color_match:
            text_color = color_match.group(1).strip()
        
        if bg_color and text_color:
            # Resolve variables
            if bg_color.startswith('var('):
                var_match = re.search(r'var\(([^)]+)\)', bg_color)
                if var_match:
                    var_name = var_match.group(1)
                    if var_name in root_vars:
                        bg_color = root_vars[var_name]
            
            if text_color.startswith('var('):
                var_match = re.search(r'var\(([^)]+)\)', text_color)
                if var_match:
                    var_name = var_match.group(1)
                    if var_name in root_vars:
                        text_color = root_vars[var_name]
            
            # Only add hex colors for now
            if bg_color.startswith('#') and text_color.startswith('#'):
                color_pairs.append((selector, bg_color, text_color))
    
    return color_pairs, root_vars

def check_contrast_ratio(color_pairs):
    """Check contrast ratios for color pairs"""
    results = []
    for selector, bg_color, text_color in color_pairs:
        try:
            bg_rgb = hex_to_rgb(bg_color)
            text_rgb = hex_to_rgb(text_color)
            
            bg_luminance = luminance(*bg_rgb)
            text_luminance = luminance(*text_rgb)
            
            ratio = contrast_ratio(bg_luminance, text_luminance)
            
            status = "PASS" if ratio >= 4.5 else "FAIL"
            if 3.0 <= ratio < 4.5:
                status = "PASS (Large Text Only)"
                
            results.append({
                "selector": selector,
                "background": bg_color,
                "text": text_color,
                "ratio": ratio,
                "status": status
            })
        except Exception as e:
            print(f"Error processing {selector}: {e}")
    
    return results

def main():
    css_file = "docker/ploughshares/static/css/custom.css"
    
    if not os.path.exists(css_file):
        print(f"CSS file not found: {css_file}")
        return
    
    print(f"Analyzing contrast ratios in {css_file}...")
    color_pairs, root_vars = parse_css_colors(css_file)
    
    print("\nCSS Variables:")
    for name, value in root_vars.items():
        print(f"  {name}: {value}")
    
    print("\nContrast Ratio Analysis:")
    results = check_contrast_ratio(color_pairs)
    
    if not results:
        print("No color pairs found for contrast analysis.")
        return
    
    # Sort by ratio (ascending)
    results.sort(key=lambda x: x["ratio"])
    
    print(f"\n{'Selector':<30} {'Background':<15} {'Text':<15} {'Ratio':<10} {'Status'}")
    print("-" * 80)
    
    for result in results:
        print(f"{result['selector']:<30} {result['background']:<15} {result['text']:<15} {result['ratio']:.2f}      {result['status']}")
    
    # Summary
    fails = sum(1 for r in results if r['status'] == 'FAIL')
    large_only = sum(1 for r in results if r['status'] == 'PASS (Large Text Only)')
    passes = sum(1 for r in results if r['status'] == 'PASS')
    
    print("\nSummary:")
    print(f"  Total color pairs analyzed: {len(results)}")
    print(f"  Passed: {passes}")
    print(f"  Passed (Large Text Only): {large_only}")
    print(f"  Failed: {fails}")
    
    print("\nRecommendations:")
    if fails > 0:
        print("  - Increase contrast for failing elements to at least 4.5:1")
    if large_only > 0:
        print("  - Ensure text with lower contrast (3:1-4.5:1) is used only for large text (18pt+)")
    print("  - Use WebAIM Contrast Checker for verification: https://webaim.org/resources/contrastchecker/")
    print("  - Test with real users, including those with visual impairments")

if __name__ == "__main__":
    main()