add save instance method (#273)

Author: Jakub Kaczmarzyk

neurodocker/generators/common.py

@@ -2,6 +2,7 @@
 
 import datetime
 import json
+import os
 import posixpath
 
 from neurodocker import __version__
@@ -101,6 +102,14 @@ class ContainerSpecGenerator:
     def render(self):
         raise NotImplementedError("not implemented")
 
+    def save(self, filepath):
+        """Save the rendered container specification to a file."""
+        if os.path.exists(filepath):
+            raise FileExistsError("File already exists: {}".format(filepath))
+        rendered = self.render()
+        with open(filepath, mode='w') as fp:
+            fp.write(rendered + '\n')
+
     @property
     def commented_header(self):
         t = datetime.datetime.utcnow().strftime("%Y-%m-%d %H:%M:%S")

neurodocker/generators/docker.py

@@ -288,9 +288,14 @@ def __init__(self, specs):
         self._prep()
         _Users.clear_memory()
 
+        self._rendered = False
+
     def render(self):
-        return self.commented_header + "\n\n".join(
-            self._ispecs_to_dockerfile_str())
+        # Cache the rendered string.
+        if not self._rendered:
+            self._rendered = self.commented_header + "\n\n".join(
+                self._ispecs_to_dockerfile_str())
+        return self._rendered
 
     def _prep(self):
         self._add_json()

neurodocker/generators/singularity.py

@@ -104,15 +104,20 @@ def __init__(self, specs):
         self._add_neurodocker_header()
         self._add_json()
 
+        self._rendered = False
+
     def render(self):
         def _render_one(section):
             renderer = getattr(self, "_render_{}".format(section))
             return renderer()
 
         if not self._parts_filled:
             self._fill_parts()
-        return self.commented_header + "\n\n".join(
-            map(_render_one, (sec for sec, con in self._order if con)))
+
+        if not self._rendered:
+            self._rendered = self.commented_header + "\n\n".join(
+                map(_render_one, (sec for sec, con in self._order if con)))
+        return self._rendered
 
     def _render_header(self):
         return "\n".join(