sayanarijit: Oops `pull_request` is missing here https://github.com/sayanarijit/expandvars/blob/115a54c6ef0b05c7e63818c49a3e9e1f3d535e84/.github/workflows/tests.yml#L2

Can you please add it and push again?
codecov-commenter: # [Codecov](https://codecov.io/gh/sayanarijit/expandvars/pull/30?src=pr&el=h1) Report
> Merging [#30](https://codecov.io/gh/sayanarijit/expandvars/pull/30?src=pr&el=desc) into [master](https://codecov.io/gh/sayanarijit/expandvars/commit/115a54c6ef0b05c7e63818c49a3e9e1f3d535e84?el=desc) will **not change** coverage.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/sayanarijit/expandvars/pull/30/graphs/tree.svg?width=650&height=150&src=pr&token=LaBTTAtjk0)](https://codecov.io/gh/sayanarijit/expandvars/pull/30?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##            master       #30   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines          455       465   +10     
=========================================
+ Hits           455       465   +10     
```


| [Impacted Files](https://codecov.io/gh/sayanarijit/expandvars/pull/30?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [expandvars.py](https://codecov.io/gh/sayanarijit/expandvars/pull/30/diff?src=pr&el=tree#diff-ZXhwYW5kdmFycy5weQ==) | `100.00% <100.00%> (ø)` | |
| [tests/test\_expandvars.py](https://codecov.io/gh/sayanarijit/expandvars/pull/30/diff?src=pr&el=tree#diff-dGVzdHMvdGVzdF9leHBhbmR2YXJzLnB5) | `100.00% <100.00%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/sayanarijit/expandvars/pull/30?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/sayanarijit/expandvars/pull/30?src=pr&el=footer). Last update [115a54c...25c0916](https://codecov.io/gh/sayanarijit/expandvars/pull/30?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).

sayanarijit: Thanks 👍
diff --git a/README.md b/README.md
index f7d6317..a1403f5 100644
--- a/README.md
+++ b/README.md
@@ -71,6 +71,16 @@ EXPANDVARS_RECOVER_NULL=foo myapp --config production.ini && echo "All fine."
 
 > WARNING: Try to avoid `export EXPANDVARS_RECOVER_NULL` because that will disable strict parsing permanently until you log out.
 
+### Customization
+
+You can customize the variable symbol and data used for the expansion by using the more general `expand` function.
+
+```python
+from expandvars import expand
+
+print(expandvars("%PATH:$HOME/bin:%{SOME_UNDEFINED_PATH:-/default/path}", environ={"PATH": "example"}, var_symbol="%"))
+# example:$HOME/bin:/default/path
+```
 
 Contributing
 ------------
diff --git a/expandvars.py b/expandvars.py
index e8b3ee8..740c3ac 100644
--- a/expandvars.py
+++ b/expandvars.py
@@ -1,6 +1,7 @@
 # -*- coding: utf-8 -*-
 
-from os import environ, getpid
+import os
+
 from io import TextIOWrapper
 
 __author__ = "Arijit Basu"
@@ -36,7 +37,7 @@ ESCAPE_CHAR = "\\"
 #
 # WARNING: Try to avoid `export EXPANDVARS_RECOVER_NULL` as it
 # will permanently disable strict parsing until you log out.
-RECOVER_NULL = environ.get("EXPANDVARS_RECOVER_NULL", None)
+RECOVER_NULL = os.environ.get("EXPANDVARS_RECOVER_NULL", None)
 
 
 class ExpandvarsException(Exception):
@@ -96,7 +97,7 @@ def _isint(val):
         return False
 
 
-def getenv(var, nounset, indirect, default=None):
+def getenv(var, nounset, indirect, environ, default=None):
     """Get value from environment variable.
 
     When nounset is True, it behaves like bash's "set -o nounset" or "set -u"
@@ -123,7 +124,7 @@ def getenv(var, nounset, indirect, default=None):
     return ""
 
 
-def escape(vars_, nounset):
+def escape(vars_, nounset, environ, var_symbol):
     """Escape the first character."""
     if len(vars_) == 0:
         raise MissingExcapedChar(vars_)
@@ -131,32 +132,46 @@ def escape(vars_, nounset):
     if len(vars_) == 1:
         return vars_[0]
 
-    if vars_[0] == "$":
-        return vars_[0] + expandvars(vars_[1:])
+    if vars_[0] == var_symbol:
+        return vars_[0] + expand(vars_[1:], environ=environ, var_symbol=var_symbol)
 
     if vars_[0] == ESCAPE_CHAR:
-        if vars_[1] == "$":
-            return ESCAPE_CHAR + expandvars(vars_[1:], nounset=nounset)
+        if vars_[1] == var_symbol:
+            return ESCAPE_CHAR + expand(
+                vars_[1:], nounset=nounset, environ=environ, var_symbol=var_symbol
+            )
         if vars_[1] == ESCAPE_CHAR:
-            return ESCAPE_CHAR + escape(vars_[2:], nounset=nounset)
+            return ESCAPE_CHAR + escape(
+                vars_[2:], nounset=nounset, environ=environ, var_symbol=var_symbol
+            )
 
-    return ESCAPE_CHAR + vars_[0] + expandvars(vars_[1:], nounset=nounset)
+    return (
+        ESCAPE_CHAR
+        + vars_[0]
+        + expand(vars_[1:], nounset=nounset, environ=environ, var_symbol=var_symbol)
+    )
 
 
-def expand_var(vars_, nounset):
+def expand_var(vars_, nounset, environ, var_symbol):
     """Expand a single variable."""
 
     if len(vars_) == 0:
-        return "$"
+        return var_symbol
 
     if vars_[0] == ESCAPE_CHAR:
-        return "$" + escape(vars_[1:], nounset=nounset)
+        return var_symbol + escape(
+            vars_[1:], nounset=nounset, environ=environ, var_symbol=var_symbol
+        )
 
-    if vars_[0] == "$":
-        return str(getpid()) + expandvars(vars_[1:], nounset=nounset)
+    if vars_[0] == var_symbol:
+        return str(os.getpid()) + expand(
+            vars_[1:], nounset=nounset, environ=environ, var_symbol=var_symbol
+        )
 
     if vars_[0] == "{":
-        return expand_modifier_var(vars_[1:], nounset=nounset)
+        return expand_modifier_var(
+            vars_[1:], nounset=nounset, environ=environ, var_symbol=var_symbol
+        )
 
     buff = []
     for c in vars_:
@@ -164,13 +179,15 @@ def expand_var(vars_, nounset):
             buff.append(c)
         else:
             n = len(buff)
-            return getenv("".join(buff), nounset=nounset, indirect=False) + expandvars(
-                vars_[n:], nounset=nounset
+            return getenv(
+                "".join(buff), nounset=nounset, indirect=False, environ=environ
+            ) + expand(
+                vars_[n:], nounset=nounset, environ=environ, var_symbol=var_symbol
             )
-    return getenv("".join(buff), nounset=nounset, indirect=False)
+    return getenv("".join(buff), nounset=nounset, indirect=False, environ=environ)
 
 
-def expand_modifier_var(vars_, nounset):
+def expand_modifier_var(vars_, nounset, environ, var_symbol):
     """Expand variables with modifier."""
 
     if len(vars_) <= 1:
@@ -189,20 +206,27 @@ def expand_modifier_var(vars_, nounset):
         elif c == "}":
             n = len(buff) + 1
             return getenv(
-                "".join(buff), nounset=nounset, indirect=indirect
-            ) + expandvars(vars_[n:], nounset=nounset)
+                "".join(buff), nounset=nounset, indirect=indirect, environ=environ
+            ) + expand(
+                vars_[n:], nounset=nounset, environ=environ, var_symbol=var_symbol
+            )
         else:
             n = len(buff)
             if c == ":":
                 n += 1
             return expand_advanced(
-                "".join(buff), vars_[n:], nounset=nounset, indirect=indirect
+                "".join(buff),
+                vars_[n:],
+                nounset=nounset,
+                indirect=indirect,
+                environ=environ,
+                var_symbol=var_symbol,
             )
 
     raise MissingClosingBrace("".join(buff))
 
 
-def expand_advanced(var, vars_, nounset, indirect):
+def expand_advanced(var, vars_, nounset, indirect, environ, var_symbol):
     """Expand substitution."""
 
     if len(vars_) == 0:
@@ -210,24 +234,42 @@ def expand_advanced(var, vars_, nounset, indirect):
 
     if vars_[0] == "-":
         return expand_default(
-            var, vars_[1:], set_=False, nounset=nounset, indirect=indirect
+            var,
+            vars_[1:],
+            set_=False,
+            nounset=nounset,
+            indirect=indirect,
+            environ=environ,
+            var_symbol=var_symbol,
         )
 
     if vars_[0] == "=":
         return expand_default(
-            var, vars_[1:], set_=True, nounset=nounset, indirect=indirect
+            var,
+            vars_[1:],
+            set_=True,
+            nounset=nounset,
+            indirect=indirect,
+            environ=environ,
+            var_symbol=var_symbol,
         )
 
     if vars_[0] == "+":
-        return expand_substitute(var, vars_[1:], nounset=nounset)
+        return expand_substitute(
+            var, vars_[1:], nounset=nounset, environ=environ, var_symbol=var_symbol
+        )
 
     if vars_[0] == "?":
-        return expand_strict(var, vars_[1:], nounset=nounset)
+        return expand_strict(
+            var, vars_[1:], nounset=nounset, environ=environ, var_symbol=var_symbol
+        )
 
-    return expand_offset(var, vars_, nounset=nounset)
+    return expand_offset(
+        var, vars_, nounset=nounset, environ=environ, var_symbol=var_symbol
+    )
 
 
-def expand_strict(var, vars_, nounset):
+def expand_strict(var, vars_, nounset, environ, var_symbol):
     """Expand variable that must be defined."""
 
     buff = []
@@ -236,9 +278,13 @@ def expand_strict(var, vars_, nounset):
             n = len(buff) + 1
             val = environ.get(var, "")
             if val:
-                return val + expandvars(vars_[n:], nounset=nounset)
+                return val + expand(
+                    vars_[n:], nounset=nounset, environ=environ, var_symbol=var_symbol
+                )
             if RECOVER_NULL is not None:
-                return RECOVER_NULL + expandvars(vars_[n:], nounset=nounset)
+                return RECOVER_NULL + expand(
+                    vars_[n:], nounset=nounset, environ=environ, var_symbol=var_symbol
+                )
             raise ParameterNullOrNotSet(var, "".join(buff) if buff else None)
 
         buff.append(c)
@@ -246,7 +292,7 @@ def expand_strict(var, vars_, nounset):
     raise MissingClosingBrace("".join(buff))
 
 
-def expand_offset(var, vars_, nounset):
+def expand_offset(var, vars_, nounset, environ, var_symbol):
     """Expand variable with offset."""
 
     buff = []
@@ -260,7 +306,14 @@ def expand_offset(var, vars_, nounset):
                 raise OperandExpected(var, offset_str)
             else:
                 offset = int(offset_str)
-            return expand_length(var, vars_[n:], offset, nounset=nounset)
+            return expand_length(
+                var,
+                vars_[n:],
+                offset,
+                nounset=nounset,
+                environ=environ,
+                var_symbol=var_symbol,
+            )
 
         if c == "}":
             n = len(buff) + 1
@@ -271,14 +324,16 @@ def expand_offset(var, vars_, nounset):
                 raise OperandExpected(var, offset_str)
             else:
                 offset = int(offset_str)
-            return getenv(var, nounset=nounset, indirect=False)[offset:] + expandvars(
-                vars_[n:], nounset=nounset
+            return getenv(var, nounset=nounset, indirect=False, environ=environ)[
+                offset:
+            ] + expand(
+                vars_[n:], nounset=nounset, environ=environ, var_symbol=var_symbol
             )
         buff.append(c)
     raise MissingClosingBrace("".join(buff))
 
 
-def expand_length(var, vars_, offset, nounset):
+def expand_length(var, vars_, offset, nounset, environ, var_symbol):
     """Expand variable with offset and length."""
 
     buff = []
@@ -302,15 +357,17 @@ def expand_length(var, vars_, offset, nounset):
             else:
                 width = offset + length
 
-            return getenv(var, nounset=nounset, indirect=False)[
+            return getenv(var, nounset=nounset, indirect=False, environ=environ)[
                 offset:width
-            ] + expandvars(vars_[n:], nounset=nounset)
+            ] + expand(
+                vars_[n:], nounset=nounset, environ=environ, var_symbol=var_symbol
+            )
 
         buff.append(c)
     raise MissingClosingBrace("".join(buff))
 
 
-def expand_substitute(var, vars_, nounset):
+def expand_substitute(var, vars_, nounset, environ, var_symbol):
     """Expand or return substitute."""
 
     sub = []
@@ -318,13 +375,17 @@ def expand_substitute(var, vars_, nounset):
         if c == "}":
             n = len(sub) + 1
             if var in environ:
-                return "".join(sub) + expandvars(vars_[n:], nounset=nounset)
-            return expandvars(vars_[n:], nounset=nounset)
+                return "".join(sub) + expand(
+                    vars_[n:], nounset=nounset, environ=environ, var_symbol=var_symbol
+                )
+            return expand(
+                vars_[n:], nounset=nounset, environ=environ, var_symbol=var_symbol
+            )
         sub.append(c)
     raise MissingClosingBrace("".join(sub))
 
 
-def expand_default(var, vars_, set_, nounset, indirect):
+def expand_default(var, vars_, set_, nounset, indirect, environ, var_symbol):
     """Expand var or return default."""
 
     default = []
@@ -335,32 +396,41 @@ def expand_default(var, vars_, set_, nounset, indirect):
             if set_ and var not in environ:
                 environ.update({var: default_})
             return getenv(
-                var, nounset=nounset, indirect=indirect, default=default_
-            ) + expandvars(vars_[n:], nounset=nounset)
+                var,
+                nounset=nounset,
+                indirect=indirect,
+                default=default_,
+                environ=environ,
+            ) + expand(
+                vars_[n:], nounset=nounset, environ=environ, var_symbol=var_symbol
+            )
 
         default.append(c)
     raise MissingClosingBrace("".join(default))
 
 
-def expandvars(vars_, nounset=False):
-    """Expand system variables Unix style.
+def expand(vars_, nounset=False, environ=os.environ, var_symbol="$"):
+    """Expand variables Unix style.
 
     Params:
-        vars_ (str): System variables to expand.
+        vars_ (str):  Variables to expand.
+        nounset (bool): If True, enables strict parsing (similar to set -u / set -o nounset in bash).
+        environ (Mapping): Elements to consider during variable expansion. Defaults to os.environ
+        var_symbol (str): Character used to identify a variable. Defaults to $
 
     Returns:
         str: Expanded values.
 
     Example usage: ::
 
-        from expandvars import expandvars
+        from expandvars import expand
 
-        print(expandvars("$PATH:$HOME/bin:${SOME_UNDEFINED_PATH:-/default/path}"))
-        # /bin:/sbin:/usr/bin:/usr/sbin:/home/you/bin:/default/path
+        print(expandvars("%PATH:$HOME/bin:%{SOME_UNDEFINED_PATH:-/default/path}", environ={"PATH": "example"}, var_symbol="%"))
+        # example:$HOME/bin:/default/path
 
         # Or
         with open(somefile) as f:
-            print(expandvars(f))
+            print(expand(f))
     """
     if isinstance(vars_, TextIOWrapper):
         # This is a file. Read it.
@@ -373,13 +443,17 @@ def expandvars(vars_, nounset=False):
 
     try:
         for c in vars_:
-            if c == "$":
+            if c == var_symbol:
                 n = len(buff) + 1
-                return "".join(buff) + expand_var(vars_[n:], nounset=nounset)
+                return "".join(buff) + expand_var(
+                    vars_[n:], nounset=nounset, environ=environ, var_symbol=var_symbol
+                )
 
             if c == ESCAPE_CHAR:
                 n = len(buff) + 1
-                return "".join(buff) + escape(vars_[n:], nounset=nounset)
+                return "".join(buff) + escape(
+                    vars_[n:], nounset=nounset, environ=environ, var_symbol=var_symbol
+                )
 
             buff.append(c)
         return "".join(buff)
@@ -389,3 +463,27 @@ def expandvars(vars_, nounset=False):
         raise MissingClosingBrace(vars_)
     except BadSubstitution:
         raise BadSubstitution(vars_)
+
+
+def expandvars(vars_, nounset=False):
+    """Expand system variables Unix style.
+
+    Params:
+        vars_ (str): System variables to expand.
+        nounset (bool): If True, enables strict parsing (similar to set -u / set -o nounset in bash).
+
+    Returns:
+        str: Expanded values.
+
+    Example usage: ::
+
+        from expandvars import expandvars
+
+        print(expandvars("$PATH:$HOME/bin:${SOME_UNDEFINED_PATH:-/default/path}"))
+        # /bin:/sbin:/usr/bin:/usr/sbin:/home/you/bin:/default/path
+
+        # Or
+        with open(somefile) as f:
+            print(expandvars(f))
+    """
+    return expand(vars_, nounset=nounset)

Support expanding using custom source data instead of os.environ
I have a use case where I would like to use the parser to expand variables in a string with respect to data specified by a custom `dict`/`defaultdict`. Something like
```python
expandvars("${ACCESS_CODE:-default_access_code}", source={"ACCESS_CODE": 123})
>>> "123"
```
This would also allow to only consider a subset of os.environ etc.
```python
filtered_environ = {k: v for k, v in os.environ.items() if k in { "DISPLAY", "NOT_EXISTING"}}
expandvars("$HOME", source=filtered_environ)
>>> ""
```
diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index 5b5e302..6304cf2 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -1,6 +1,5 @@
 name: Run Tests
-on: push
-
+on: [push, pull_request]
 jobs:
   pytest:
     name: pytest
diff --git a/tests/test_expandvars.py b/tests/test_expandvars.py
index eaf9b1d..49f5e21 100644
--- a/tests/test_expandvars.py
+++ b/tests/test_expandvars.py
@@ -300,3 +300,31 @@ def test_invalid_operand_err():
             repr(o)
         )
         assert isinstance(e.value, expandvars.OperandExpected)
+
+
+@pytest.mark.parametrize("var_symbol", ["%", "&", "£", "="])
+def test_expand_var_symbol(var_symbol):
+    assert (
+        expandvars.expand(
+            var_symbol + "{FOO}", environ={"FOO": "test"}, var_symbol=var_symbol
+        )
+        == "test"
+    )
+    assert (
+        expandvars.expand(var_symbol + "FOO", environ={}, var_symbol=var_symbol) == ""
+    )
+    assert (
+        expandvars.expand(
+            var_symbol + "{FOO:-default_value}", environ={}, var_symbol=var_symbol
+        )
+        == "default_value"
+    )
+    with pytest.raises(expandvars.ParameterNullOrNotSet):
+        expandvars.expand(var_symbol + "{FOO:?}", environ={}, var_symbol=var_symbol)
+
+    assert (
+        expandvars.expand(
+            var_symbol + "{FOO},$HOME", environ={"FOO": "test"}, var_symbol=var_symbol
+        )
+        == "test,$HOME"
+    )

pip install -e .
parse_log_pytest
requirements.txt
pytest pytest-cov
apt-get update apt-get install -y gcc
3.9
requirements/base.txt
pytest --no-header -rA --tb=line --color=no -p no:cacheprovider -W ignore::DeprecationWarning
coverage==7.8.0
exceptiongroup==1.2.2
-e git+https://github.com/sayanarijit/expandvars.git@115a54c6ef0b05c7e63818c49a3e9e1f3d535e84#egg=expandvars
iniconfig==2.1.0
packaging==24.2
pluggy==1.5.0
pytest==8.3.5
pytest-cov==6.0.0
tomli==2.2.1

name: expandvars
channels:
  - defaults
  - https://repo.anaconda.com/pkgs/main
  - https://repo.anaconda.com/pkgs/r
  - conda-forge
dependencies:
  - _libgcc_mutex=0.1=main
  - _openmp_mutex=5.1=1_gnu
  - ca-certificates=2025.2.25=h06a4308_0
  - ld_impl_linux-64=2.40=h12ee557_0
  - libffi=3.4.4=h6a678d5_1
  - libgcc-ng=11.2.0=h1234567_1
  - libgomp=11.2.0=h1234567_1
  - libstdcxx-ng=11.2.0=h1234567_1
  - ncurses=6.4=h6a678d5_0
  - openssl=3.0.16=h5eee18b_0
  - pip=25.0=py39h06a4308_0
  - python=3.9.21=he870216_1
  - readline=8.2=h5eee18b_0
  - setuptools=75.8.0=py39h06a4308_0
  - sqlite=3.45.3=h5eee18b_0
  - tk=8.6.14=h39e8969_0
  - tzdata=2025a=h04d1e81_0
  - wheel=0.45.1=py39h06a4308_0
  - xz=5.6.4=h5eee18b_1
  - zlib=1.2.13=h5eee18b_1
  - pip:
      - coverage==7.8.0
      - exceptiongroup==1.2.2
      - expandvars==0.0.0
      - iniconfig==2.1.0
      - packaging==24.2
      - pluggy==1.5.0
      - pytest==8.3.5
      - pytest-cov==6.0.0
      - tomli==2.2.1
prefix: /opt/conda/envs/expandvars

tests/test_expandvars.py::test_expand_var_symbol[%] tests/test_expandvars.py::test_expand_var_symbol[&] tests/test_expandvars.py::test_expand_var_symbol[\xa3] tests/test_expandvars.py::test_expand_var_symbol[=]
tests/test_expandvars.py::test_expandvars_constant tests/test_expandvars.py::test_expandvars_empty tests/test_expandvars.py::test_expandvars_simple tests/test_expandvars.py::test_expandvars_from_file tests/test_expandvars.py::test_expandvars_combo tests/test_expandvars.py::test_expandvars_pid tests/test_expandvars.py::test_expandvars_get_default tests/test_expandvars.py::test_expandvars_update_default tests/test_expandvars.py::test_expandvars_substitute tests/test_expandvars.py::test_offset tests/test_expandvars.py::test_offset_length tests/test_expandvars.py::test_expandvars_indirection tests/test_expandvars.py::test_escape tests/test_expandvars.py::test_corner_cases tests/test_expandvars.py::test_strict_parsing tests/test_expandvars.py::test_missing_escapped_character tests/test_expandvars.py::test_invalid_length_err tests/test_expandvars.py::test_bad_substitution_err tests/test_expandvars.py::test_brace_never_closed_err tests/test_expandvars.py::test_invalid_operand_err
README.md expandvars.py
README.md expandvars.py