Add regression test and comment for negative increments in loops (Closes #219)

Author: fabianp

Makefile

@@ -20,6 +20,7 @@ test:
 	$(PYTHON) test/test_mprof.py
 	$(PYTHON) test/test_async.py
 	$(PYTHON) test/test_ipython.py
+	$(PYTHON) test/test_increment_display.py
 	mprof run test/test_func.py
 
 develop:

memory_profiler.py

@@ -680,6 +680,9 @@ def trace(self, code, lineno, prev_lineno):
         previous_inc = prev_value[0] if prev_value else 0
 
         prev_line_value = self[code].get(prev_lineno, None) if prev_lineno else None
+        # Use the most recent memory of the previous line (index 3) rather than
+        # its historical maximum (index 1) so loops do not accumulate false
+        # negative increments when memory drops below an earlier peak (#219).
         prev_line_memory = prev_line_value[3] if prev_line_value else 0
         occ_count = self[code][lineno][2] + 1 if lineno in self[code] else 1
         self[code][lineno] = (

test/test_increment_display.py

@@ -1,3 +1,4 @@
+import tracemalloc
 import unittest
 
 from memory_profiler import LineProfiler, profile, show_results
@@ -7,13 +8,19 @@
 class TestIncrementDisplay(unittest.TestCase):
     """Tests memory incrementation / decrementation display"""
 
+    def setUp(self):
+        tracemalloc.start()
+
+    def tearDown(self):
+        tracemalloc.stop()
+
     def test_loop_count(self):
 
         def some_loop():
             for i in range(12):  # line -2
                 a = 1            # line -1
 
-        profiler = LineProfiler()
+        profiler = LineProfiler(backend='tracemalloc')
         wrapped = profiler(some_loop)
         wrapped()
         show_results(profiler)
@@ -28,7 +35,7 @@ def test_normal_incr(self):
         def normal_incr():
             use_some_memory = [1] * (10 ** 6)
 
-        profiler = LineProfiler()
+        profiler = LineProfiler(backend='tracemalloc')
         wrapped = profiler(normal_incr)
         wrapped()
 
@@ -48,7 +55,7 @@ def loop_incr():
                 c = [2] * (2 * 10 ** 7)  # line -2
                 a.append(c)
 
-        profiler = LineProfiler()
+        profiler = LineProfiler(backend='tracemalloc')
         wrapped = profiler(loop_incr)
         wrapped()
 
@@ -64,7 +71,7 @@ def del_stuff():
             b = [2] * (2 * 10 ** 7)
             del b
 
-        profiler = LineProfiler()
+        profiler = LineProfiler(backend='tracemalloc')
         wrapped = profiler(del_stuff)
         wrapped()
 
@@ -76,6 +83,35 @@ def del_stuff():
         self.assertGreater(del_line[1], 0)
         self.assertAlmostEqual(-del_line[0], b_line[0], delta=1)
 
+    def test_no_false_negative_incr_after_peak_in_loop(self):
+        """Regression test for Issue #219 / PR #350.
+
+        When a line inside a loop reaches a high memory peak on an early
+        iteration that is later freed, subsequent iterations of the following
+        line must compare against the previous line's most recent memory,
+        not its historical maximum memory (which would accumulate bogus
+        negative increments on every iteration).
+        """
+        def loop_with_early_peak():
+            for i in range(5):
+                # Peaks on i == 0 (~150 MiB), small on i > 0
+                x = [1] * (2 * 10 ** 7 if i == 0 else 10)
+                y = 1  # Must NOT accumulate a ~-150 MiB decrement on i = 1..4
+                del x
+
+        profiler = LineProfiler(backend='tracemalloc')
+        wrapped = profiler(loop_with_early_peak)
+        wrapped()
+
+        show_results(profiler)
+        measures = list(profiler.code_map.values())[0]
+        # Ordered lines: for, x = ..., y = 1, del x
+        y_line = [measures[lineno] for lineno in sorted(measures)][-2]
+        y_inc = y_line[0]
+        # Before PR #350, y_inc was around -600 MiB (4 * -150 MiB).
+        # With PR #350, y = 1 has ~0 MiB increment.
+        self.assertGreater(y_inc, -5.0)
+
 
 if __name__ == '__main__':
     unittest.main()