python with echo

[hong zhang]

List,

I have a question of python using echo.

POWER = 14
return_value = os.system('echo 14 > /sys/class/net/wlan1/device/tx_power')

can assign 14 to tx_power

But 
return_value = os.system('echo $POWER > /sys/class/net/wlan1/device/tx_power')

return_value is 256 not 0. It cannot assign 14 to tx_power.

What problem is it?

os.system("echo $POWER") returns 0 but
os.system("echo $POWER > /sys/class/net/wlan1/device/tx_power") returns 256.

Appreciate any help!

Thanks.

---henry