Use time.monotonic() for python>=3.3 to ensure no negative time delta…

…s due to changed system time. Will default back to time.time() if time.monotonic() is not available. Fixes #1.

.gitignore

@@ -8,3 +8,5 @@ build/
 .pytest_cache/
 .tox/
 *.pyc
+.idea/
+.envrc

simple_pid/PID.py

@@ -1,4 +1,5 @@
 import time
+import warnings
 
 
 def _clamp(value, limits):
@@ -12,6 +13,15 @@ def _clamp(value, limits):
     return value
 
 
+try:
+    # get monotonic time to ensure that time deltas are always positive
+    _current_time = time.monotonic
+except AttributeError:
+    # time.monotonic() not available (using python < 3.3), fallback to time.time()
+    _current_time = time.time
+    warnings.warn('time.monotonic() not available, using time.time() as fallback. Consider using Python 3.3 or newer to get monotonic time measurements.')
+
+
 class PID(object):
     """
     A simple PID controller. No fuss.
@@ -37,7 +47,7 @@ def __init__(self, Kp=1.0, Ki=0.0, Kd=0.0, setpoint=0, sample_time=0.01, output_
 
         self._error_sum = 0
 
-        self._last_time = time.time()
+        self._last_time = _current_time()
         self._last_output = None
         self._proportional = 0
         self._last_input = None
@@ -50,8 +60,9 @@ def __call__(self, input_):
         """
         if not self.auto_mode:
             return self._last_output
-
-        dt = time.time() - self._last_time
+
+        now = _current_time()
+        dt = now - self._last_time
 
         if self.sample_time is not None and dt < self.sample_time and self._last_output is not None:
             # only update every sample_time seconds
@@ -81,7 +92,7 @@ def __call__(self, input_):
         # keep track of state
         self._last_output = output
         self._last_input = input_
-        self._last_time = time.time()
+        self._last_time = now
 
         return output
 

tests/test_pid.py

@@ -1,4 +1,4 @@
-
+import sys
 import time
 import pytest
 from simple_pid import PID
@@ -99,6 +99,15 @@ def test_sample_time():
     assert pid(100) == control
 
 
+def test_monotonic():
+    from simple_pid.PID import _current_time
+
+    if sys.version_info < (3, 3):
+        assert _current_time == time.time
+    else:
+        assert _current_time == time.monotonic
+
+
 def test_clamp():
     from simple_pid.PID import _clamp