From 694eecd3699253881c6a33ef9caa367a213bc7a0 Mon Sep 17 00:00:00 2001
From: HansBug <hansbug@buaa.edu.cn>
Date: Tue, 24 May 2022 13:34:50 +0800
Subject: [PATCH 1/2] dev(hansbug): add default value to env template

---
 hbutils/string/template.py   | 27 +++++++++++++++++++++++----
 test/string/test_template.py |  4 ++++
 2 files changed, 27 insertions(+), 4 deletions(-)

diff --git a/hbutils/string/template.py b/hbutils/string/template.py
index ad38b5ddc85..d5253c49cea 100644
--- a/hbutils/string/template.py
+++ b/hbutils/string/template.py
@@ -3,12 +3,19 @@
     Useful utilities for template a string.
 """
 from string import Template
-from typing import Optional, Mapping
+from typing import Optional, Mapping, Any
 
-__all__ = ['env_template']
+from ..design import SingletonMark
 
+__all__ = [
+    'env_template'
+]
 
-def env_template(template: str, environ: Optional[Mapping[str, str]] = None, safe: bool = False) -> str:
+_NO_DEFAULT_VALUE = SingletonMark('_NO_DEFAULT_VALUE')
+
+
+def env_template(template: str, environ: Optional[Mapping[str, Any]] = None,
+                 safe: bool = False, default: Any = _NO_DEFAULT_VALUE) -> str:
     """
     Overview:
         Mapping all the environment values (not system environment variables) into a template string.
@@ -18,6 +25,8 @@ def env_template(template: str, environ: Optional[Mapping[str, str]] = None, saf
         - environ (:obj:`Optional[Mapping[str, str]]`): Environment values, should be a mapping.
         - safe (:obj:`bool`): Safe substitute, default is ``False`` which means \
             all the value used in template should be able to found in the given ``environ``.
+        - default (:obj:`Any`): Default value when no variable provided. Default is ``_NO_DEFAULT_VALUE``, which \
+            means ``KeyError`` will be raised.
 
     Returns:
         - result (:obj:`str`): Substituted string.
@@ -31,6 +40,16 @@ def env_template(template: str, environ: Optional[Mapping[str, str]] = None, saf
         >>> env_template('${A} + 1 = ${B}', {'A': '1'}, safe=True)
         '1 + 1 = ${B}'
     """
+
+    class _DefaultDict(dict):
+        def __getitem__(self, item):
+            return dict.get(self, item, default)
+
     _template = Template(template)
+    env = environ or {}
+    if default is not _NO_DEFAULT_VALUE:
+        env = _DefaultDict(env)
+
     _func = _template.safe_substitute if safe else _template.substitute
-    return _func(**(environ or {}))
+    # noinspection PyArgumentList
+    return _func(env)
diff --git a/test/string/test_template.py b/test/string/test_template.py
index d174b026580..1e1d3de9b8e 100644
--- a/test/string/test_template.py
+++ b/test/string/test_template.py
@@ -10,3 +10,7 @@ def test_env_template(self):
         assert env_template('this is the ${number} day.', dict(number='1st')) == 'this is the 1st day.'
         with pytest.raises(KeyError):
             env_template('this is the ${number} day')
+
+    def test_env_template_with_default(self):
+        assert env_template('${A} + 1 == ${B}', {'A': 1}, default='') == '1 + 1 == '
+        assert env_template('${A} + 1 == ${B}', {'A': 1}, default='', safe=True) == '1 + 1 == '

From f915b6c9b5d8077a6e8d2506f5f40bc9b4b9ae0c Mon Sep 17 00:00:00 2001
From: HansBug <hansbug@buaa.edu.cn>
Date: Tue, 24 May 2022 13:48:43 +0800
Subject: [PATCH 2/2] doc(hansbug): add comments for default argument

---
 hbutils/string/template.py | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/hbutils/string/template.py b/hbutils/string/template.py
index d5253c49cea..18cdfab131f 100644
--- a/hbutils/string/template.py
+++ b/hbutils/string/template.py
@@ -39,6 +39,8 @@ def env_template(template: str, environ: Optional[Mapping[str, Any]] = None,
         KeyError: 'B'
         >>> env_template('${A} + 1 = ${B}', {'A': '1'}, safe=True)
         '1 + 1 = ${B}'
+        >>> env_template('${A} + 1 = ${B}', {'A': '1'}, default='')  # like environment variable
+        '1 + 1 = '
     """
 
     class _DefaultDict(dict):