BUG: Motor method 'export_eng' for liquid motors bug fix. (solves #473)

CHANGELOG.md

@@ -71,6 +71,7 @@ You can install this version by running `pip install rocketpy==1.2.0`
 
 ### Fixed
 
+- BUG: export_eng 'Motor' method would not work for liquid motors. [#559](https://github.com/RocketPy-Team/RocketPy/pull/559)
 - BUG: Update flight trajectory plot axes limits [#552](https://github.com/RocketPy-Team/RocketPy/pull/552)
 - BUG: fix `get_controller_observed_variables` in the air brakes examples [#551](https://github.com/RocketPy-Team/RocketPy/pull/551)
 - MNT: small fixes before v1.2 [#550](https://github.com/RocketPy-Team/RocketPy/pull/550)

rocketpy/motors/motor.py

@@ -1012,31 +1012,36 @@
         None
         """
         # Open file
-        file = open(file_name, "w")
-
-        # Write first line
-        file.write(
-            motor_name
-            + " {:3.1f} {:3.1f} 0 {:2.3} {:2.3} RocketPy\n".format(
-                2000 * self.grain_outer_radius,
-                1000
-                * self.grain_number
-                * (self.grain_initial_height + self.grain_separation),
-                self.propellant_initial_mass,
-                self.propellant_initial_mass,
-            )
-        )
+        with open(file_name, "w") as file:
+            # Write first line
+            def get_attr_value(obj, attr_name, multiplier=1):
+                return multiplier * getattr(obj, attr_name, 0)
+
+            grain_outer_radius = get_attr_value(self, "grain_outer_radius", 2000)
+            grain_number = get_attr_value(self, "grain_number", 1000)
+            grain_initial_height = get_attr_value(self, "grain_initial_height")
+            grain_separation = get_attr_value(self, "grain_separation")
+
+            grain_total = grain_number * (grain_initial_height + grain_separation)
+
+            if grain_outer_radius == 0 or grain_total == 0:
+                warnings.warn(
+                    "The motor object doesn't have some grain-related attributes. "
+                    "Using zeros to write to file."
+                )
 
-        # Write thrust curve data points
-        for time, thrust in self.thrust.source[1:-1, :]:
-            # time, thrust = item
-            file.write("{:.4f} {:.3f}\n".format(time, thrust))
+            file.write(
+                f"{motor_name} {grain_outer_radius:3.1f} {grain_total:3.1f} 0 "
+                f"{self.propellant_initial_mass:2.3} "
+                f"{self.propellant_initial_mass:2.3} RocketPy\n"
+            )
 
-        # Write last line
-        file.write("{:.4f} {:.3f}\n".format(self.thrust.source[-1, 0], 0))
+            # Write thrust curve data points
+            for time, thrust in self.thrust.source[1:-1, :]:
+                file.write(f"{time:.4f} {thrust:.3f}\n")
 
-        # Close file
-        file.close()
+            # Write last line
+            file.write(f"{self.thrust.source[-1, 0]:.4f} {0:.3f}\n")
 
         return None