Create files in w, w+, a, and a+ modes when calling File.new and the file is missing.

lib/fakefs/file.rb

@@ -108,7 +108,7 @@ module FakeFS
       @file = FileSystem.find(path)
       @open = true
 
-      check_file_existence! if !file_creation_mode?
+      file_creation_mode? ? create_missing_file : check_file_existence!
     end
 
     def close
@@ -134,10 +134,7 @@ module FakeFS
       raise IOError, 'closed stream' unless @open
       raise IOError, 'not open for writing' if read_only?
 
-      if !File.exists?(@path)
-        @file = FileSystem.add(path, FakeFile.new)
-      end
-
+      create_missing_file
       @file.content += content
     end
     alias_method :print, :write
@@ -170,5 +167,11 @@ module FakeFS
         raise ArgumentError, "illegal access mode #{mode}"
       end
     end
+
+    def create_missing_file
+      if !File.exists?(@path)
+        @file = FileSystem.add(path, FakeFile.new)
+      end
+    end
   end
 end

test/fakefs_test.rb

@@ -126,6 +126,26 @@ class FakeFSTest < Test::Unit::TestCase
     end
   end
 
+  def test_creates_files_in_write_only_mode
+    File.open("foo", "w")
+    assert File.exists?("foo")
+  end
+
+  def test_creates_files_in_read_write_truncate_mode
+    File.open("foo", "w+")
+    assert File.exists?("foo")
+  end
+
+  def test_creates_files_in_append_write_only
+    File.open("foo", "a")
+    assert File.exists?("foo")
+  end
+
+  def test_creates_files_in_append_read_write
+    File.open("foo", "a+")
+    assert File.exists?("foo")
+  end
+
   def test_can_read_files_once_written
     path = '/path/to/file.txt'
     File.open(path, 'w') do |f|