Allow to ignore env variable substitution default value

Author: dolfinus

README.md

@@ -498,7 +498,9 @@ You can use these substitutions in `template`, `dev_template` or `dirty_template
 
 - `{env:SOMEVAR}`: Value of environment variable `SOMEVAR`. Examples:
 
-  - You can pass default value using `{env:SOMEVAR:default}` syntax.  Default value for missing variables is `UNKNOWN`.
+  - You can pass default value using `{env:SOMEVAR:default}` syntax.
+
+  - Default value for missing variables is `UNKNOWN`. If you need to just skip the substitution, use `{env:SOMEVAR:IGNORE}` syntax.
 
   - It is possible to pass another substitution instead of a default value using `{env:SOMEVAR:{subst}}` syntax, e.g. `{env:BUILD_NUMBER:{ccount}}`.
 

setuptools_git_versioning.py

@@ -139,7 +139,12 @@ def read_version_from_file(path):  # type: (Union[str, os.PathLike]) -> str
 def subst_env_variables(template):  # type: (str) -> str
     if "{env" in template:
         for var, default in ENV_VARS_REGEXP.findall(template):
-            value = os.environ.get(var, default or "UNKNOWN")
+            if default.upper() == "IGNORE":
+                default = ""
+            elif not default:
+                default = "UNKNOWN"
+
+            value = os.environ.get(var, default)
             template, _ = ENV_VARS_REGEXP.subn(value, template, count=1)
 
     return template