Python Import not being recognized

Question

Hi guys,

I'm currently working through a textbook call Programming Robots with ROS. There is a chapter that goes over a simple movement code for a turtlebot simulator. The code is pasted below.

 # BEGIN ALL
#!/usr/bin/env python
import rospy
from geometry_msgs.msg import Twist

cmd_vel_pub = rospy.Publisher('cmd_vel', Twist, queue_size=1) #<1>
rospy.init_node('red_light_green_light')

red_light_twist = Twist() #<2>
green_light_twist = Twist()
green_light_twist.linear.x = 0.5 #<3>

driving_forward = False
light_change_time = rospy.Time.now()
rate = rospy.Rate(10)

while not rospy.is_shutdown():
  if driving_forward:
    cmd_vel_pub.publish(green_light_twist) #<4>
  else:
    cmd_vel_pub.publish(red_light_twist)
  # BEGIN PART_1
  if rospy.Time.now() > light_change_time: #<5>
    driving_forward = not driving_forward
    light_change_time = rospy.Time.now() + rospy.Duration(3)
  # END PART_1
  rate.sleep() #<6>
# END ALL

Nothing seems to be wrong with the code itself. However, when I run

chmod +x red_light_green_light.py

followed by

./red_light_green_light.py cmd_vel:=cmd_vel_mux/input/teleop

I receive the following errors:

./red_light_green_light.py: line 3: import: command not found
from: can't read /var/mail/geometry_msgs.msg
./red_light_green_light.py: line 6: syntax error near unexpected token `('
./red_light_green_light.py: line 6: `cmd_vel_pub = rospy.Publisher('cmd_vel', Twist, queue_size=1) #<1>'

I'm not clear to the second and third error, but the firs error is claiming to not be able to find import rospy. Any assistance would be greatly appreciated.

Cheers,

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Originally posted by Starmit on ROS Answers with karma: 1 on 2016-07-24

Post score: 0

Answer 1

The first error is import: command not found; this means that the import command itself is unrecognized; not the library being imported. This usually happens when you try to run a python program with the shell.

When you execute a program with ./, it looks at the first line, and if it starts with #!, it uses the rest of the line as the program for executing the file. If the file doesn't start with a #!, it's executed with the default shell. It looks like that's what happing here.

I would double-check your file and make sure that the very first line is #!/usr/bin/env python

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Originally posted by ahendrix with karma: 47576 on 2016-07-24

This answer was ACCEPTED on the original site

Post score: 5

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Original comments

Comment by Airuno2L on 2016-07-25:
Yep, delete the line # BEGIN ALL and make sure #!/usr/bin/env python is the first text in the file.