diff --git a/.travis.yml b/.travis.yml
index 8f51de3..8ccd240 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -33,6 +33,7 @@ script:
   - tox
 
 before_install:
+  - pip install --upgrade pip
   - pip install coveralls
 
 after_success:
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 56a7a94..effa251 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,7 +7,7 @@ project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
 
-_There are no unreleased changes at this time._
+- Fix resolution order in variable expansion with `override=False` (#? by [@bbc2]).
 
 ## [0.15.0] - 2020-10-28
 
diff --git a/README.md b/README.md
index 5c9aeaf..36f3b2b 100644
--- a/README.md
+++ b/README.md
@@ -41,13 +41,22 @@ export SECRET_KEY=YOURSECRETKEYGOESHERE
 
 Python-dotenv can interpolate variables using POSIX variable expansion.
 
-The value of a variable is the first of the values defined in the following list:
+With `load_dotenv(override=True)` or `dotenv_values()`, the value of a variable is the
+first of the values defined in the following list:
 
 - Value of that variable in the `.env` file.
 - Value of that variable in the environment.
 - Default value, if provided.
 - Empty string.
 
+With `load_dotenv(override=False)`, the value of a variable is the first of the values
+defined in the following list:
+
+- Value of that variable in the environment.
+- Value of that variable in the `.env` file.
+- Default value, if provided.
+- Empty string.
+
 Ensure that variables are surrounded with `{}` like `${HOME}` as bare
 variables such as `$HOME` are not expanded.
 
diff --git a/src/dotenv/main.py b/src/dotenv/main.py
index 58a23f3..b366b18 100644
--- a/src/dotenv/main.py
+++ b/src/dotenv/main.py
@@ -4,7 +4,6 @@ from __future__ import absolute_import, print_function, unicode_literals
 import io
 import logging
 import os
-import re
 import shutil
 import sys
 import tempfile
@@ -13,13 +12,13 @@ from contextlib import contextmanager
 
 from .compat import IS_TYPE_CHECKING, PY2, StringIO, to_env
 from .parser import Binding, parse_stream
+from .variables import parse_variables
 
 logger = logging.getLogger(__name__)
 
 if IS_TYPE_CHECKING:
-    from typing import (
-        Dict, Iterable, Iterator, Match, Optional, Pattern, Union, Text, IO, Tuple
-    )
+    from typing import (IO, Dict, Iterable, Iterator, Mapping, Optional, Text,
+                        Tuple, Union)
     if sys.version_info >= (3, 6):
         _PathLike = os.PathLike
     else:
@@ -30,18 +29,6 @@ if IS_TYPE_CHECKING:
     else:
         _StringIO = StringIO[Text]
 
-__posix_variable = re.compile(
-    r"""
-    \$\{
-        (?P<name>[^\}:]*)
-        (?::-
-            (?P<default>[^\}]*)
-        )?
-    \}
-    """,
-    re.VERBOSE,
-)  # type: Pattern[Text]
-
 
 def with_warn_for_invalid_lines(mappings):
     # type: (Iterator[Binding]) -> Iterator[Binding]
@@ -56,13 +43,14 @@ def with_warn_for_invalid_lines(mappings):
 
 class DotEnv():
 
-    def __init__(self, dotenv_path, verbose=False, encoding=None, interpolate=True):
-        # type: (Union[Text, _PathLike, _StringIO], bool, Union[None, Text], bool) -> None
+    def __init__(self, dotenv_path, verbose=False, encoding=None, interpolate=True, override=True):
+        # type: (Union[Text, _PathLike, _StringIO], bool, Union[None, Text], bool, bool) -> None
         self.dotenv_path = dotenv_path  # type: Union[Text,_PathLike, _StringIO]
         self._dict = None  # type: Optional[Dict[Text, Optional[Text]]]
         self.verbose = verbose  # type: bool
         self.encoding = encoding  # type: Union[None, Text]
         self.interpolate = interpolate  # type: bool
+        self.override = override  # type: bool
 
     @contextmanager
     def _get_stream(self):
@@ -83,13 +71,14 @@ class DotEnv():
         if self._dict:
             return self._dict
 
+        raw_values = self.parse()
+
         if self.interpolate:
-            values = resolve_nested_variables(self.parse())
+            self._dict = OrderedDict(resolve_variables(raw_values, override=self.override))
         else:
-            values = OrderedDict(self.parse())
+            self._dict = OrderedDict(raw_values)
 
-        self._dict = values
-        return values
+        return self._dict
 
     def parse(self):
         # type: () -> Iterator[Tuple[Text, Optional[Text]]]
@@ -98,13 +87,13 @@ class DotEnv():
                 if mapping.key is not None:
                     yield mapping.key, mapping.value
 
-    def set_as_environment_variables(self, override=False):
-        # type: (bool) -> bool
+    def set_as_environment_variables(self):
+        # type: () -> bool
         """
         Load the current dotenv as system environemt variable.
         """
         for k, v in self.dict().items():
-            if k in os.environ and not override:
+            if k in os.environ and not self.override:
                 continue
             if v is not None:
                 os.environ[to_env(k)] = to_env(v)
@@ -217,27 +206,26 @@ def unset_key(dotenv_path, key_to_unset, quote_mode="always"):
     return removed, key_to_unset
 
 
-def resolve_nested_variables(values):
-    # type: (Iterable[Tuple[Text, Optional[Text]]]) -> Dict[Text, Optional[Text]]
-    def _replacement(name, default):
-        # type: (Text, Optional[Text]) -> Text
-        default = default if default is not None else ""
-        ret = new_values.get(name, os.getenv(name, default))
-        return ret  # type: ignore
+def resolve_variables(values, override):
+    # type: (Iterable[Tuple[Text, Optional[Text]]], bool) -> Mapping[Text, Optional[Text]]
 
-    def _re_sub_callback(match):
-        # type: (Match[Text]) -> Text
-        """
-        From a match object gets the variable name and returns
-        the correct replacement
-        """
-        matches = match.groupdict()
-        return _replacement(name=matches["name"], default=matches["default"])  # type: ignore
+    new_values = {}  # type: Dict[Text, Optional[Text]]
 
-    new_values = {}
+    for (name, value) in values:
+        if value is None:
+            result = None
+        else:
+            atoms = parse_variables(value)
+            env = {}  # type: Dict[Text, Optional[Text]]
+            if override:
+                env.update(os.environ)  # type: ignore
+                env.update(new_values)
+            else:
+                env.update(new_values)
+                env.update(os.environ)  # type: ignore
+            result = "".join(atom.resolve(env) for atom in atoms)
 
-    for (k, v) in values:
-        new_values[k] = __posix_variable.sub(_re_sub_callback, v) if v is not None else None
+        new_values[name] = result
 
     return new_values
 
@@ -316,10 +304,11 @@ def load_dotenv(dotenv_path=None, stream=None, verbose=False, override=False, in
                   Defaults to `False`.
     """
     f = dotenv_path or stream or find_dotenv()
-    return DotEnv(f, verbose=verbose, interpolate=interpolate, **kwargs).set_as_environment_variables(override=override)
+    dotenv = DotEnv(f, verbose=verbose, interpolate=interpolate, override=override, **kwargs)
+    return dotenv.set_as_environment_variables()
 
 
 def dotenv_values(dotenv_path=None, stream=None, verbose=False, interpolate=True, **kwargs):
     # type: (Union[Text, _PathLike, None], Optional[_StringIO], bool, bool, Union[None, Text]) -> Dict[Text, Optional[Text]]  # noqa: E501
     f = dotenv_path or stream or find_dotenv()
-    return DotEnv(f, verbose=verbose, interpolate=interpolate, **kwargs).dict()
+    return DotEnv(f, verbose=verbose, interpolate=interpolate, override=True, **kwargs).dict()
diff --git a/src/dotenv/variables.py b/src/dotenv/variables.py
new file mode 100644
index 0000000..4828dfc
--- /dev/null
+++ b/src/dotenv/variables.py
@@ -0,0 +1,106 @@
+import re
+from abc import ABCMeta
+
+from .compat import IS_TYPE_CHECKING
+
+if IS_TYPE_CHECKING:
+    from typing import Iterator, Mapping, Optional, Pattern, Text
+
+
+_posix_variable = re.compile(
+    r"""
+    \$\{
+        (?P<name>[^\}:]*)
+        (?::-
+            (?P<default>[^\}]*)
+        )?
+    \}
+    """,
+    re.VERBOSE,
+)  # type: Pattern[Text]
+
+
+class Atom():
+    __metaclass__ = ABCMeta
+
+    def __ne__(self, other):
+        # type: (object) -> bool
+        result = self.__eq__(other)
+        if result is NotImplemented:
+            return NotImplemented
+        return not result
+
+    def resolve(self, env):
+        # type: (Mapping[Text, Optional[Text]]) -> Text
+        raise NotImplementedError
+
+
+class Literal(Atom):
+    def __init__(self, value):
+        # type: (Text) -> None
+        self.value = value
+
+    def __repr__(self):
+        # type: () -> str
+        return "Literal(value={})".format(self.value)
+
+    def __eq__(self, other):
+        # type: (object) -> bool
+        if not isinstance(other, self.__class__):
+            return NotImplemented
+        return self.value == other.value
+
+    def __hash__(self):
+        # type: () -> int
+        return hash((self.__class__, self.value))
+
+    def resolve(self, env):
+        # type: (Mapping[Text, Optional[Text]]) -> Text
+        return self.value
+
+
+class Variable(Atom):
+    def __init__(self, name, default):
+        # type: (Text, Optional[Text]) -> None
+        self.name = name
+        self.default = default
+
+    def __repr__(self):
+        # type: () -> str
+        return "Variable(name={}, default={})".format(self.name, self.default)
+
+    def __eq__(self, other):
+        # type: (object) -> bool
+        if not isinstance(other, self.__class__):
+            return NotImplemented
+        return (self.name, self.default) == (other.name, other.default)
+
+    def __hash__(self):
+        # type: () -> int
+        return hash((self.__class__, self.name, self.default))
+
+    def resolve(self, env):
+        # type: (Mapping[Text, Optional[Text]]) -> Text
+        default = self.default if self.default is not None else ""
+        result = env.get(self.name, default)
+        return result if result is not None else ""
+
+
+def parse_variables(value):
+    # type: (Text) -> Iterator[Atom]
+    cursor = 0
+
+    for match in _posix_variable.finditer(value):
+        (start, end) = match.span()
+        name = match.groupdict()["name"]
+        default = match.groupdict()["default"]
+
+        if start > cursor:
+            yield Literal(value=value[cursor:start])
+
+        yield Variable(name=name, default=default)
+        cursor = end
+
+    length = len(value)
+    if cursor < length:
+        yield Literal(value=value[cursor:length])

diff --git a/tests/test_main.py b/tests/test_main.py
index 6b9458d..b927d7f 100644
--- a/tests/test_main.py
+++ b/tests/test_main.py
@@ -257,6 +257,28 @@ def test_load_dotenv_existing_variable_override(dotenv_file):
     assert os.environ == {"a": "b"}
 
 
+@mock.patch.dict(os.environ, {"a": "c"}, clear=True)
+def test_load_dotenv_redefine_var_used_in_file_no_override(dotenv_file):
+    with open(dotenv_file, "w") as f:
+        f.write('a=b\nd="${a}"')
+
+    result = dotenv.load_dotenv(dotenv_file)
+
+    assert result is True
+    assert os.environ == {"a": "c", "d": "c"}
+
+
+@mock.patch.dict(os.environ, {"a": "c"}, clear=True)
+def test_load_dotenv_redefine_var_used_in_file_with_override(dotenv_file):
+    with open(dotenv_file, "w") as f:
+        f.write('a=b\nd="${a}"')
+
+    result = dotenv.load_dotenv(dotenv_file, override=True)
+
+    assert result is True
+    assert os.environ == {"a": "b", "d": "b"}
+
+
 @mock.patch.dict(os.environ, {}, clear=True)
 def test_load_dotenv_utf_8():
     stream = StringIO("a=à")
diff --git a/tests/test_variables.py b/tests/test_variables.py
new file mode 100644
index 0000000..86b0646
--- /dev/null
+++ b/tests/test_variables.py
@@ -0,0 +1,35 @@
+import pytest
+
+from dotenv.variables import Literal, Variable, parse_variables
+
+
+@pytest.mark.parametrize(
+    "value,expected",
+    [
+        ("", []),
+        ("a", [Literal(value="a")]),
+        ("${a}", [Variable(name="a", default=None)]),
+        ("${a:-b}", [Variable(name="a", default="b")]),
+        (
+            "${a}${b}",
+            [
+                Variable(name="a", default=None),
+                Variable(name="b", default=None),
+            ],
+        ),
+        (
+            "a${b}c${d}e",
+            [
+                Literal(value="a"),
+                Variable(name="b", default=None),
+                Literal(value="c"),
+                Variable(name="d", default=None),
+                Literal(value="e"),
+            ],
+        ),
+    ]
+)
+def test_parse_variables(value, expected):
+    result = parse_variables(value)
+
+    assert list(result) == expected

Inconsistency in priorities of variable expansion
# Behavior
python-dotenv priorities are correctly documented, but inconsistent when loading a .env file with internal dependencies in variable expansion.

Consider the following `.env` file:
```
HOSTNAME=localhost:8080
URL=http://${HOSTNAME}
```

Currently, if we set the `HOSTNAME` environment variable to `localhost:8081`, then run `load_dotenv()`, then the environment ends up with `HOSTNAME=localhost:8081` and `URL=http://localhost8080`, which at first is surprising.

# Underlying reason

The reason for this is that (by default) `load_dotenv()` does not override set environment variables, but when doing variable expansion inside a given file, `load_dotenv`prefers the variable as set in the file than from the environment (as documented in https://github.com/theskumar/python-dotenv#usages).

# Workaround

A workaround consists of changing the priorities in the file explicitly, as in this example:
```
HOSTNAME=${HOSTNAME:-localhost:8080}
URL=http://${HOSTNAME}
```

# Suggestion

I would suggest that the variable expansion priority logic follow the same behavior as the general "overriding or not" behavior. More precisely:

- when `override==False`, variable expansion should follow the following priorities:
  - Value of that variable in the environment.
  - Value of that variable in the .env file.
  - Default value, if provided.
  - Empty string.
- when `override==True`, variable expansion should follow the currently documented priorities:
  - Value of that variable in the .env file.
  - Value of that variable in the environment.
  - Default value, if provided.
  - Empty string.

It would consist of a breaking change in behavior, but it seems that in the case it does matter, the current behavior is probably not what is expected.

Is that a change you'd be willing to accept? 
tests/test_main.py::test_set_key_no_file tests/test_main.py::test_set_key[-a--expected0-a=""\n] tests/test_main.py::test_set_key[-a-b-expected1-a="b"\n] tests/test_main.py::test_set_key[-a-'b'-expected2-a="b"\n] tests/test_main.py::test_set_key[-a-"b"-expected3-a="b"\n] tests/test_main.py::test_set_key[-a-b'c-expected4-a="b'c"\n] tests/test_main.py::test_set_key[-a-b"c-expected5-a="b\\"c"\n] tests/test_main.py::test_set_key[a=b-a-c-expected6-a="c"\n] tests/test_main.py::test_set_key[a=b\n-a-c-expected7-a="c"\n] tests/test_main.py::test_set_key[a=b\n\n-a-c-expected8-a="c"\n\n] tests/test_main.py::test_set_key[a=b\nc=d-a-e-expected9-a="e"\nc=d] tests/test_main.py::test_set_key[a=b\nc=d\ne=f-c-g-expected10-a=b\nc="g"\ne=f] tests/test_main.py::test_set_key[a=b\n-c-d-expected11-a=b\nc="d"\n] tests/test_main.py::test_get_key_no_file tests/test_main.py::test_get_key_not_found tests/test_main.py::test_get_key_ok tests/test_main.py::test_get_key_none tests/test_main.py::test_unset_with_value tests/test_main.py::test_unset_no_value tests/test_main.py::test_unset_non_existent_file tests/test_main.py::test_find_dotenv_no_file_raise tests/test_main.py::test_find_dotenv_no_file_no_raise tests/test_main.py::test_find_dotenv_found tests/test_main.py::test_load_dotenv_existing_file tests/test_main.py::test_load_dotenv_no_file_verbose tests/test_main.py::test_load_dotenv_existing_variable_no_override tests/test_main.py::test_load_dotenv_existing_variable_override tests/test_main.py::test_load_dotenv_redefine_var_used_in_file_no_override tests/test_main.py::test_load_dotenv_redefine_var_used_in_file_with_override tests/test_main.py::test_load_dotenv_utf_8 tests/test_main.py::test_load_dotenv_in_current_dir tests/test_main.py::test_dotenv_values_file tests/test_main.py::test_dotenv_values_stream[env0-a=$b-False-expected0] tests/test_main.py::test_dotenv_values_stream[env1-a=$b-True-expected1] tests/test_main.py::test_dotenv_values_stream[env2-a=${b}-False-expected2] tests/test_main.py::test_dotenv_values_stream[env3-a=${b}-True-expected3] tests/test_main.py::test_dotenv_values_stream[env4-a=${b:-d}-False-expected4] tests/test_main.py::test_dotenv_values_stream[env5-a=${b:-d}-True-expected5] tests/test_main.py::test_dotenv_values_stream[env6-b=c\na=${b}-True-expected6] tests/test_main.py::test_dotenv_values_stream[env7-a=${b}-True-expected7] tests/test_main.py::test_dotenv_values_stream[env8-a=${b:-d}-True-expected8] tests/test_main.py::test_dotenv_values_stream[env9-a="${b}"-True-expected9] tests/test_main.py::test_dotenv_values_stream[env10-a='${b}'-True-expected10] tests/test_main.py::test_dotenv_values_stream[env11-a=x${b}y-True-expected11] tests/test_main.py::test_dotenv_values_stream[env12-a=${a}-True-expected12] tests/test_main.py::test_dotenv_values_stream[env13-a=${a}-True-expected13] tests/test_main.py::test_dotenv_values_stream[env14-a=${a:-c}-True-expected14] tests/test_main.py::test_dotenv_values_stream[env15-a=${a:-c}-True-expected15] tests/test_main.py::test_dotenv_values_stream[env16-a=${b}${b}-True-expected16] tests/test_main.py::test_dotenv_values_stream[env17-b=d\na=${b}-True-expected17] tests/test_main.py::test_dotenv_values_stream[env18-a=b\na=c\nd=${a}-True-expected18] tests/test_main.py::test_dotenv_values_stream[env19-a=b\nc=${a}\nd=e\nc=${d}-True-expected19] tests/test_variables.py::test_parse_variables[-expected0] tests/test_variables.py::test_parse_variables[a-expected1] tests/test_variables.py::test_parse_variables[${a}-expected2] tests/test_variables.py::test_parse_variables[${a:-b}-expected3] tests/test_variables.py::test_parse_variables[${a}${b}-expected4] tests/test_variables.py::test_parse_variables[a${b}c${d}e-expected5]