Reuse virtual env of prev deployment

Author: Saipriya-1144

src/BuildScriptGenerator/Python/PythonBashBuildSnippet.sh.tpl

@@ -65,6 +65,46 @@ fi
     # Execute the resolved CreateVenvCommand
     echo "Executing: $CreateVenvCommand"
     $CreateVenvCommand
+
+    CACHE_EXTRACTION_DIR=""/tmp/oryx_cache_$(date +%s)""
+    OLD_VENV_RESTORED=false
+
+    if [ -f "/home/site/wwwroot/output.tar.gz" ]; then
+        echo "Found previous deployment, checking for virtual environment reuse..."
+
+        # Create temporary extraction directory
+        mkdir -p "$CACHE_EXTRACTION_DIR"
+
+        # Extract previous deployment to temp location
+        echo "Extracting previous deployment..."
+        tar -xzf /home/site/wwwroot/output.tar.gz -C "$CACHE_EXTRACTION_DIR"
+
+        # Compare requirements.txt files
+        OLD_REQUIREMENTS="$CACHE_EXTRACTION_DIR/$REQUIREMENTS_TXT_FILE"
+        CURRENT_REQUIREMENTS="$REQUIREMENTS_TXT_FILE"
+
+        if [ -f "$OLD_REQUIREMENTS" ] && [ -f "$CURRENT_REQUIREMENTS" ]; then
+            if cmp -s "$OLD_REQUIREMENTS" "$CURRENT_REQUIREMENTS"; then
+                echo "Requirements.txt unchanged - reusing existing virtual environment"
+                
+                # Check if old virtual environment exists
+                if [ -d "$CACHE_EXTRACTION_DIR/$VIRTUALENVIRONMENTNAME" ]; then
+                    echo "Copying cached virtual environment..."
+                    # Remove current empty venv and replace with old one
+                    cp -r "$CACHE_EXTRACTION_DIR/$VIRTUALENVIRONMENTNAME" ./
+                    
+                    # Reactivate the copied environment
+                    source $VIRTUALENVIRONMENTNAME/bin/activate
+                    
+                    OLD_VENV_RESTORED=true
+                    echo "Successfully restored virtual environment from cache"
+                    echo "Skipping pip install - using cached packages"
+                fi
+            else
+                echo "Requirements.txt changed - performing incremental update"
+            fi
+        fi
+    fi
 
     echo Activating virtual environment...
     printf %s " , $ActivateVenvCommand" >> "$COMMAND_MANIFEST_FILE"