Fixed the test about formatting, and improved the error message slightly

Author: Sylvain MARIE

mini_lambda/main.py

@@ -412,7 +412,9 @@ def __format__(self, *args):
         """
         raise FunctionDefinitionError('__format__ is not supported by _LambdaExpression, since python raises an'
                                       ' error when its output is not directly an object of the type it expects.'
-                                      'Please either use the equivalent x.format() method, or the Format method provided'
+                                      'Please either use the equivalent x.format() method if your variable is the '
+                                      'string template, or the `Format` (for `format`) or `Str.format` '
+                                      '(for `str.format`) methods provided'
                                       ' at mini_lambda package level.If you did not use __format__ in your expression, '
                                       'you probably used a standard method such as math.log(x) instead of a method '
                                       ' converted to mini_lambda such as Log(x). Please check the documentation for '

mini_lambda/tests/test_mini_lambda.py

@@ -182,24 +182,32 @@ def test_evaluator_bytes():
 def test_evaluator_format():
     """ Representable Object : tests that format() works """
 
-    s = InputVar('s', str)
+    # (1) the variable is the template
+    s_tpl = InputVar('s_tpl', str)
 
     # the str operator cannot be overloaded
-    formatted_string = s.format('yes')
-    formatted_string = formatted_string.as_function()
+    format_my_yes = s_tpl.format('yes')
+    format_my_yes = format_my_yes.as_function()
+
+    assert format_my_yes('{} sure') == 'yes sure'
 
-    assert formatted_string('{}') == 'yes'
+    # (2) the variable is the value
+    s = InputVar('s', str)
 
     # the str operator cannot be overloaded
     with pytest.raises(FunctionDefinitionError):
         '{} {}'.format(s, s)
 
     # so we provide this equivalent
-    reasonable_string = Format('{} {}', s, s)
+    reasonable_string = Str.format('{} {}', s, s)
     reasonable_string = reasonable_string.as_function()
 
     assert reasonable_string('hello') == 'hello hello'
 
+    # and there is also a Format method replacing `format`
+    stringify = Format(s).as_function()
+    assert stringify(12) == '12'
+
 
 def test_evaluator_sizeof():
     """ Object : tests that sys.getsizeof() raises the correct error message and that the equivalent Getsizeof()