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A ReportLab Flowable subclass for embedding signature images in PDFs with automatic fallback to placeholder text when images are unavailable or cannot be loaded.

Purpose

SignatureImage extends ReportLab's Flowable class to provide robust signature image handling in PDF generation. It attempts to load and render a signature image from a file path, and gracefully falls back to displaying placeholder rectangles with descriptive text ('Signature Unavailable' or 'No Signature on File') when the image file doesn't exist or cannot be loaded. This ensures PDF generation continues successfully even when signature images are missing, making it ideal for automated document generation workflows where signature availability may vary.

Source Code

class SignatureImage(Flowable):
    """
    Custom flowable for signature images in ReportLab
    
    This class allows embedding signature images in PDFs with fallback
    placeholders if the image can't be loaded.
    """
    
    def __init__(self, img_path: str, width: float = 2*inch, height: float = 0.75*inch):
        """
        Initialize the signature image flowable
        
        Parameters
        ----------
        img_path : str
            Path to the signature image
        width : float
            Width of the signature image
        height : float
            Height of the signature image
        """
        Flowable.__init__(self)
        self.img_path = img_path
        self.width = width
        self.height = height
    
    def draw(self):
        """Draw the signature image or a placeholder if the image can't be loaded"""
        if os.path.exists(self.img_path):
            try:
                img = Image(self.img_path, self.width, self.height)
                img.drawOn(self.canv, 0, 0)
            except Exception as e:
                # Draw a placeholder if image can't be loaded
                self.canv.setStrokeColor(colors.grey)
                self.canv.rect(0, 0, self.width, self.height)
                self.canv.setFont("Helvetica", 8)
                self.canv.drawString(5, self.height/2, "Signature Unavailable")
                logger.warning(f"Could not load signature image: {str(e)}")
        else:
            # Draw a placeholder if file doesn't exist
            self.canv.setStrokeColor(colors.grey)
            self.canv.rect(0, 0, self.width, self.height)
            self.canv.setFont("Helvetica", 8)
            self.canv.drawString(5, self.height/2, "No Signature on File")
            logger.warning(f"Signature image not found: {self.img_path}")

Parameters

Parameter Details

img_path: String path to the signature image file. Can be absolute or relative path. The class checks for file existence before attempting to load. Supports image formats compatible with ReportLab's Image class (PNG, JPEG, GIF, etc.).

width: Float value specifying the width of the signature image in ReportLab units. Defaults to 2 inches (2*inch). This width is used both for rendering the image and for the placeholder rectangle if the image cannot be loaded.

height: Float value specifying the height of the signature image in ReportLab units. Defaults to 0.75 inches (0.75*inch). This height is used both for rendering the image and for the placeholder rectangle if the image cannot be loaded.

Return Value

Instantiation returns a SignatureImage object that is a Flowable, which can be added to ReportLab document templates (like SimpleDocTemplate) and will be rendered when the PDF is built. The draw() method does not return a value but renders content directly to the canvas (self.canv) provided by the ReportLab framework.

Class Interface

Methods

Attributes

Dependencies

• reportlab
• os
• logging

Required Imports

import os
import logging
from reportlab.lib import colors
from reportlab.lib.units import inch
from reportlab.platypus import Image
from reportlab.platypus import Flowable

Usage Example

from reportlab.platypus import SimpleDocTemplate, Flowable
from reportlab.lib.units import inch
from reportlab.lib import colors
from reportlab.platypus import Image
import os
import logging

logger = logging.getLogger(__name__)

# Create a SignatureImage instance with default dimensions
sig_image = SignatureImage('path/to/signature.png')

# Create with custom dimensions
sig_image_custom = SignatureImage(
    img_path='signatures/john_doe.png',
    width=3*inch,
    height=1*inch
)

# Add to a ReportLab document
doc = SimpleDocTemplate('document.pdf')
story = []
story.append(sig_image)
doc.build(story)

# The draw() method is called automatically by ReportLab during PDF generation
# You typically don't call it directly

Best Practices

• Always ensure the logger is properly configured before using this class to capture warning messages about missing or unloadable signature images
• Use ReportLab units (inch, cm) for width and height parameters to ensure consistent sizing across different PDF configurations
• The class is designed to be added to ReportLab story lists and should not have its draw() method called directly - let the ReportLab framework handle rendering
• Verify signature image paths are correct and accessible before PDF generation to avoid placeholder rendering, though the class handles missing files gracefully
• Consider the aspect ratio when setting custom width and height to avoid distorting signature images
• The class logs warnings but does not raise exceptions, ensuring PDF generation continues even with missing signatures - monitor logs for signature availability issues
• Signature images should be in formats supported by ReportLab's Image class (PNG, JPEG recommended for best quality)
• The placeholder text is positioned at height/2, so very small heights may result in text being cut off or poorly positioned

Tags

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