Defaultable variable substitution

Author: nedbat

CHANGES.rst

@@ -17,9 +17,10 @@ Change history for Coverage.py
 Unreleased
 ----------
 
-- Environment variable substitution in configuration files can now be strict:
-  using a question mark suffix like ``${VARNAME?}`` will raise an error if
-  ``VARNAME`` is not defined as an environment variable.
+- Environment variable substitution in configuration files now supports two
+  syntaxes for controlling the behavior of undefined variables: if ``VARNAME``
+  is not defined, ``${VARNAME?}`` will raise an error, and ``${VARNAME-default
+  value}`` will use "default value".
 
 
 .. _changes_50a2:

coverage/misc.py

@@ -258,7 +258,8 @@ def substitute_variables(text, variables=os.environ):
 
         $VAR
         ${VAR}
-        ${VAR?}     strict: an error if VAR isn't defined.
+        ${VAR?}             strict: an error if VAR isn't defined.
+        ${VAR-missing}      defaulted: "missing" if VAR isn't defined.
 
     A dollar can be inserted with ``$$``.
 
@@ -280,16 +281,20 @@ def dollar_replace(m):
                 if word not in variables:
                     msg = "Variable {} is undefined: {}".format(word, text)
                     raise CoverageException(msg)
-            return variables.get(word, '')
+            return variables.get(word, m.group('defval') or '')
 
     dollar_pattern = r"""(?x)   # Use extended regex syntax
         \$(?:                   # A dollar sign, then
         (?P<v1>\w+) |           #   a plain word,
+        (?P<char>\$) |          #   or a dollar sign.
         {                       #   or a {-wrapped word,
             (?P<v2>\w+)
-            (?P<strict>\??)     # with maybe a strict marker
-        } |
-        (?P<char>[$])           #   or a dollar sign.
+            (?:
+            (?P<strict>\?)      #       with a strict marker
+            |
+            -(?P<defval>[^}]*)  #       or a default value
+            )?
+        }
         )
         """
     text = re.sub(dollar_pattern, dollar_replace, text)

doc/config.rst

@@ -59,10 +59,17 @@ or ``0`` and are case-insensitive.
 
 Environment variables can be substituted in by using dollar signs: ``$WORD``
 or ``${WORD}`` will be replaced with the value of ``WORD`` in the environment.
-A dollar sign can be inserted with ``$$``.  If you want to raise an error if
-an environment variable is undefined, use a question mark suffix: ``${WORD?}``.
-Otherwise, missing environment variables will result in empty strings with no
-error.
+A dollar sign can be inserted with ``$$``.  Special forms can be used to
+control what happens if the variable isn't defined in the environment:
+
+- If you want to raise an error if an environment variable is undefined, use a
+  question mark suffix: ``${WORD?}``.
+
+- If you want to provide a default for missing variables, use a dash with a
+  default value: ``${WORD-default value}``.
+
+- Otherwise, missing environment variables will result in empty strings with no
+  error.
 
 Many sections and values correspond roughly to commands and options in
 the :ref:`command-line interface <cmd>`.

tests/test_misc.py

@@ -152,6 +152,7 @@ def test_format_lines(statements, lines, result):
     ("Multiple: $$ $FOO $BAR ${FOO}", "Multiple: $ fooey xyzzy fooey"),
     ("Ill-formed: ${%5} ${{HI}} ${", "Ill-formed: ${%5} ${{HI}} ${"),
     ("Strict: ${FOO?} is there", "Strict: fooey is there"),
+    ("Defaulted: ${WUT-missing}!", "Defaulted: missing!"),
 ])
 def test_substitute_variables(before, after):
     assert substitute_variables(before, VARS) == after