bugfix create existing folder

examples/simple_example_city_railway_generator.py

@@ -9,7 +9,9 @@ from flatland.envs.rail_generators_city_generator import city_generator
 from flatland.envs.schedule_generators import city_schedule_generator
 from flatland.utils.rendertools import RenderTool, AgentRenderVariant
 
-os.mkdir("./../render_output/")
+OUTPUT_DIR = "./../render_output/"
+if not os.path.exists(OUTPUT_DIR):
+    os.mkdir(OUTPUT_DIR)
 
 for itrials in np.arange(1, 15, 1):
     print(itrials, "generate new city")
@@ -52,7 +54,7 @@ for itrials in np.arange(1, 15, 1):
     # store rendered file into render_output if the path exists
     env_renderer.gl.save_image(
         os.path.join(
-            "./../render_output/",
+            OUTPUT_DIR,
             "flatland_frame_{:04d}.png".format(itrials)
         ))