Change substitution format for env variables

Author: dolfinus

README.md

@@ -498,9 +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`.
 
-  - It is possible to pass another substitution instead of a default value using `{env:SOMEVAR:subst}` syntax, e.g. `{env:BUILD_NUMBER:ccount}`.
+  - It is possible to pass another substitution instead of a default value using `{env:SOMEVAR:{subst}}` syntax, e.g. `{env:BUILD_NUMBER:{ccount}}`.
 
 - `{timestamp:format}`: Current timestamp rendered into a specified format. Examples:
 

setuptools_git_versioning.py

@@ -15,7 +15,7 @@
 DEFAULT_DEV_TEMPLATE = "{tag}.post{ccount}+git.{sha}"  # type: str
 DEFAULT_DIRTY_TEMPLATE = "{tag}.post{ccount}+git.{sha}.dirty"  # type: str
 DEFAULT_STARTING_VERSION = "0.0.1"
-ENV_VARS_REGEXP = re.compile(r"\{env:([^:}]+):?([^}]+)?\}", re.IGNORECASE | re.UNICODE)  # type: Pattern
+ENV_VARS_REGEXP = re.compile(r"\{env:([^:}]+):?([^}]+}?)?\}", re.IGNORECASE | re.UNICODE)  # type: Pattern
 TIMESTAMP_REGEXP = re.compile(r"\{timestamp:?([^:}]+)?\}", re.IGNORECASE | re.UNICODE)  # type: Pattern
 
 
@@ -139,12 +139,8 @@ 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)
-
-            if value:
-                template, _ = ENV_VARS_REGEXP.subn(value, template, count=1)
-            else:
-                template, _ = ENV_VARS_REGEXP.subn("{" + (default or "UNKNOWN") + "}", template, count=1)
+            value = os.environ.get(var, default or "UNKNOWN")
+            template, _ = ENV_VARS_REGEXP.subn(value, template, count=1)
 
     return template