render only modified template files

.gitignore

@@ -0,0 +1 @@
+.last_run

README.md

@@ -5,9 +5,25 @@ _Automatic code generation from Jinja2 templates_
 Generate your code by running the following command:
 
 ```bash
-python3 ./src/StempelWerk.py PATH_TO_SETTINGS_FILE
+python3 ./src/StempelWerk.py [--only-modified] PATH_TO_SETTINGS_FILE
 ```
 
+## Performance
+
+By default, StempelWerk renders all template files located in the
+specified template directory.
+
+When you use the command line argument `--only-modified`, however,
+StempelWerk tries to process only the template files that have changed
+since the last successful run. Use of this command line argument is
+highly discouraged in CI/CD pipelines!
+
+_This logic is not infallible: some file systems update modification
+times in a weird manner, and changes to master templates (called
+"stencils" in StempelWerk) are currently not handled. However, in such
+a case you can simply use StempelWerk without the `--only-modified`
+argument._
+
 ## Settings
 
 Settings for StempelWerk are provided in the form of a JSON file. The
@@ -32,10 +48,9 @@ directory.
 
 ### `stencil_dir_name`
 
-Name of directory that contains master templates (called "stencils" in
-StempelWerk). All files in `template_dir/stencil_dir_name` will be
-loaded into Jinja2 and can be referenced from other templates at
-runtime.
+Name of directory that contains stencils. All files in
+`template_dir/stencil_dir_name` will be loaded into Jinja2 and can be
+referenced from other templates at runtime.
 
 In addition, files in any directory matching this name will not be
 rendered.

process_templates.sh

@@ -1,2 +1,4 @@
 #! /usr/bin/bash
+
+# process all templates
 python3 "./src/StempelWerk.py" ../settings_example.json

src/StempelWerk.py

@@ -41,7 +41,9 @@
 #
 # ----------------------------------------------------------------------------
 
+import datetime
 import json
+import math
 import os
 import pathlib
 import platform
@@ -54,12 +56,14 @@
 from DirWalk.DirWalk import dirwalk
 
 
-VERSION = '0.3.2'
-
 # ensure that this script can be called from anywhere
 script_dir = os.path.dirname(os.path.abspath(__file__))
 os.chdir(script_dir)
 
+VERSION = '0.4.0'
+LAST_RUN_FILE = os.path.normpath(
+    os.path.join(script_dir, '../.last_run'))
+
 
 # Auto-create settings class to write leaner code
 #
@@ -190,7 +194,7 @@ def render_template(settings, cached_templates, template_filename):
     print()
 
 
-def process_templates(settings_path):
+def process_templates(settings_path, only_modified=False):
     settings = load_settings(settings_path)
 
     # do not end entries with path separators ("/" or "\")!
@@ -206,13 +210,31 @@ def process_templates(settings_path):
     # load and cache stencils
     cached_templates = cache_templates(settings, list_templates=False)
 
+    modified_after = None
+    if only_modified:
+        try:
+            # get time of last run
+            with open(LAST_RUN_FILE) as f:
+                modified_after = f.read().strip()
+        except IOError:
+            modified_after = None
+
     # find all Jinja2 files in template directory
     for template_filename in dirwalk(
             settings.template_dir,
             included=inclusions,
-            modified_after=None):
+            modified_after=modified_after):
         render_template(settings, cached_templates, template_filename)
 
+    # save time of current run
+    with open(LAST_RUN_FILE, mode='w') as f:
+        # round down to ensure that files with inaccurate timestamps and
+        # other edge cases are included
+        current_timestamp = math.floor(
+            datetime.datetime.now().timestamp())
+
+        f.write(str(current_timestamp))
+
 
 def display_version():
     print()
@@ -231,7 +253,18 @@ def display_version():
 
     display_version()
 
+    command_line_arguments = list(sys.argv)
+    only_modified = False
+
+    # extremely primitive command line parsing
+    if '--only-modified' in command_line_arguments:
+        only_modified = True
+        command_line_arguments.remove('--only-modified')
+
+    # highly sophisticated command line parsing
+    settings_path = command_line_arguments.pop()
+
     # settings path is relative to the path of this script
-    settings_path = os.path.join(script_dir, sys.argv[1])
+    settings_path = os.path.join(script_dir, settings_path)
 
-    process_templates(settings_path)
+    process_templates(settings_path, only_modified)

update_templates.sh

@@ -0,0 +1,4 @@
+#! /usr/bin/bash
+
+# process only templates modified since the last run
+python3 "./src/StempelWerk.py" --only-modified ../settings_example.json