Add support for logarithm in templates (home-assistant#10824)

* Add support for logarithm in templates

This adds a 'log' filter that takes the logarithm of the given value,
with an optional base number. The base defaults to 'e' - the natural
logarithm

* Remove usage of log10 in template filter 'log'

* Add logarithm as a global

This makes it possible to write:
'{{ log(4, 2) }}'

homeassistant/helpers/template.py

@@ -4,6 +4,7 @@
 import logging
 import random
 import re
+import math
 
 import jinja2
 from jinja2 import contextfilter
@@ -423,6 +424,14 @@ def multiply(value, amount):
         return value
 
 
+def logarithm(value, base=math.e):
+    """Filter to get logarithm of the value with a spesific base."""
+    try:
+        return math.log(float(value), float(base))
+    except (ValueError, TypeError):
+        return value
+
+
 def timestamp_custom(value, date_format=DATE_STR_FORMAT, local=True):
     """Filter to convert given timestamp to format."""
     try:
@@ -508,13 +517,15 @@ def is_safe_callable(self, obj):
 ENV = TemplateEnvironment()
 ENV.filters['round'] = forgiving_round
 ENV.filters['multiply'] = multiply
+ENV.filters['log'] = logarithm
 ENV.filters['timestamp_custom'] = timestamp_custom
 ENV.filters['timestamp_local'] = timestamp_local
 ENV.filters['timestamp_utc'] = timestamp_utc
 ENV.filters['is_defined'] = fail_when_undefined
 ENV.filters['max'] = max
 ENV.filters['min'] = min
 ENV.filters['random'] = random_every_time
+ENV.globals['log'] = logarithm
 ENV.globals['float'] = forgiving_float
 ENV.globals['now'] = dt_util.now
 ENV.globals['utcnow'] = dt_util.utcnow

tests/helpers/test_template.py

@@ -3,6 +3,7 @@
 from datetime import datetime
 import unittest
 import random
+import math
 from unittest.mock import patch
 
 from homeassistant.components import group
@@ -125,6 +126,29 @@ def test_multiply(self):
                 template.Template('{{ %s | multiply(10) | round }}' % inp,
                                   self.hass).render())
 
+    def test_logarithm(self):
+        """Test logarithm."""
+        tests = [
+            (4, 2, '2.0'),
+            (1000, 10, '3.0'),
+            (math.e, '', '1.0'),
+            ('"invalid"', '_', 'invalid'),
+            (10, '"invalid"', '10.0'),
+        ]
+
+        for value, base, expected in tests:
+            self.assertEqual(
+                expected,
+                template.Template(
+                    '{{ %s | log(%s) | round(1) }}' % (value, base),
+                    self.hass).render())
+
+            self.assertEqual(
+                expected,
+                template.Template(
+                    '{{ log(%s, %s) | round(1) }}' % (value, base),
+                    self.hass).render())
+
     def test_strptime(self):
         """Test the parse timestamp method."""
         tests = [