Python: setting value of attribute

Question

In this API, it says the following regarding setting a motor's polarity:

polarity

Sets the polarity of the motor. With normal polarity, a positive duty cycle will cause the motor to rotate clockwise. With inversed polarity, a positive duty cycle will cause the motor to rotate counter-clockwise. Valid values are normal and inversed.

import ev3dev.ev3 as ev3
import numpy as numpy
m = ev3.LargeMotor('outA')
time = numpy.random.normal(loc=5.0, scale=1.0, size=None)
time = time * 1000
speed = 180 / ( time / 1000 )
m.run_timed(time_sp=time, speed_sp=speed)
m.polarity = 'inverse'
time = numpy.random.normal(loc=5.0, scale=1.0, size=None)
time = time * 1000
speed = 180 / ( time / 1000 )
m.run_timed(time_sp=5000, speed_sp=100)

m.polarity = 'inverse' doesn't work:

robot@ev3dev:~$ python gaussian_motor.py 
Traceback (most recent call last):
  File "gaussian_motor.py", line 8, in <module>
    m.polarity = 'inverse'
  File "/usr/lib/python2.7/dist-packages/ev3dev/core.py", line 388, in polarity
    self.set_attr_string('polarity', value)
  File "/usr/lib/python2.7/dist-packages/ev3dev/core.py", line 216, in set_attr_string
    self._set_attribute(attribute, "{0}".format(value))
  File "/usr/lib/python2.7/dist-packages/ev3dev/core.py", line 204, in _set_attribute
    self._attribute_cache.write(abspath(self._path + '/' + attribute), value)
  File "/usr/lib/python2.7/dist-packages/ev3dev/core.py", line 91, in write
    f.write(value.encode())
IOError: [Errno 22] Invalid argument

How am I supposed to do this?

Patrick Haugh · Accepted Answer · 2016-10-20 13:25:10Z

1

Valid values are normal and inversed. (Emphasis mine)

You forgot the d.

Also it looks like polarity is a method for setting the polarity. You may need to access it like motor.polarity('inversed')

answered Oct 20, 2016 at 13:25

Patrick Haugh

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2 Comments

chepner Over a year ago

No, that documentation would have listed it as polarity(...) if it was a method.

Patrick Haugh Over a year ago

@chepner you're right. I was confused because a lot of them use words usually associated with functions, like return

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