Fix variable precedence

This patch fixes the variable precedence in order to have the default
values read last:

1. Values from CLI
2. Values from the environment
3. Default values

Tests were added accordingly.

Drive-by:
* Update `.gitignore` file
* Update `tox.ini` to skip missing environments

Fixes: pallets#873

.gitignore

@@ -8,6 +8,7 @@ dist
 build
 docs/_build
 click.egg-info
+venv/
 .tox
 .cache
 .ropeproject

click/core.py

@@ -1328,12 +1328,13 @@ def get_default(self, ctx):
     def add_to_parser(self, parser, ctx):
         pass
 
+
     def consume_value(self, ctx, opts):
         value = opts.get(self.name)
-        if value is None:
-            value = ctx.lookup_default(self.name)
         if value is None:
             value = self.value_from_envvar(ctx)
+        if value is None:
+            value = ctx.lookup_default(self.name)
         return value
 
     def type_cast_value(self, ctx, value):

tests/test_commands.py

@@ -1,6 +1,8 @@
 # -*- coding: utf-8 -*-
 import re
+
 import click
+import pytest
 
 
 def test_other_command_invoke(runner):
@@ -267,3 +269,49 @@ def foo_bar():
     result = runner.invoke(cli, ['foo-bar'])
     assert not result.exception
     assert result.output.splitlines() == ['foo-bar']
+
+
+def test_environment_variables(runner):
+    @click.group()
+    def cli():
+        pass
+
+    @cli.command()
+    @click.option('--name', envvar='CLICK_NAME')
+    def foo(name):
+        click.echo(name)
+
+    result = runner.invoke(cli, ['foo'], env={'CLICK_NAME': 'environment'})
+
+    assert not result.exception
+    assert result.output == 'environment\n'
+
+
+# Ensures the variables are read in the following order:
+# 1. CLI
+# 2. Environment
+# 3. Defaults
+variable_precedence_testdata = [
+    (['foo', '--name=cli'], {'CLICK_NAME': 'environment'}, 'cli\n'),
+    (['foo'], {'CLICK_NAME': 'environment'}, 'environment\n'),
+    (['foo'], None, 'defaults\n'),
+]
+
+
+@pytest.mark.parametrize("command,environment,expected",
+                          variable_precedence_testdata)
+def test_variable_precendence_00(runner, command, environment, expected):
+    @click.group()
+    def cli():
+        pass
+
+    @cli.command()
+    @click.option('--name', envvar='CLICK_NAME')
+    def foo(name):
+        click.echo(name)
+
+    defaults = {'foo': {'name': 'defaults'}}
+    result = runner.invoke(cli, command, default_map=defaults, env=environment)
+
+    assert not result.exception
+    assert result.output == expected

tox.ini

@@ -1,5 +1,6 @@
 [tox]
 envlist = py27,py34,py35,py36,pypy
+skip_missing_interpreters = true
 
 [testenv]
 passenv = LANG