Running python script in Blender

1. Open a Text Editor view in Blender.
2. Press Alt + O, or go to Text>Open Text Block and open the .py file
3. Then simply press Run script :D

P.s. Instead of opening a file in step 2, you can also hit the "+ New" button and create a new script instead.

Note : In newer versions the Run Script button label has been replaced with a Play icon :

You can also execute the following code in the python console to execute an external script without opening it up in the text editor:

filename = "/full/path/to/myscript.py"
exec(compile(open(filename).read(), filename, 'exec'))

The above code comes from the following link:

Blender - Tips and Tricks

this answer is too late, but to help anyone with the same problem

via the terminal:

blender yourblendfilenameorpath --python drawcar.py 

from the man pages

       -P or --python <filename>
              Run the given Python script file.

To run a script by another script or from console:

import bpy

script = bpy.data.texts["script_name.py"]
exec(script.as_string())