diff --git a/PyTeX/build/build/pytex_file.py b/PyTeX/build/build/pytex_file.py
index 340f193..e84b15d 100644
--- a/PyTeX/build/build/pytex_file.py
+++ b/PyTeX/build/build/pytex_file.py
@@ -6,6 +6,7 @@ from PyTeX.format.formatterif import FormatterIF
 from PyTeX.build.build.enums import PyTeXFileType
 from .hashing import md5
 from ...format.formatting_config import FormattingConfig
+from ...format.auto_format import formatter_from_file_extension
 
 
 class PyTeXSourceFile:
@@ -13,15 +14,22 @@ class PyTeXSourceFile:
             self,
             relative_path: RelativePath,
             formatter: Optional[FormatterIF] = None,
+            default_config: Optional[FormattingConfig] = None,
             pytex_file_type: Optional[PyTeXFileType] = None
     ):
         self._relative_path: RelativePath = relative_path
-        self._pytex_file_type: Optional[PyTeXFileType] = None
-        self._formatter: Optional[FormatterIF] = None
+        if formatter is not None:
+            self._formatter: FormatterIF = formatter
+        else:
+            self._formatter = formatter_from_file_extension(
+                relative_path.path,
+                config=default_config,
+                locate_file_config=True,
+                allow_infile_config=True
+            )
+        self._pytex_file_type: Optional[PyTeXFileType] = pytex_file_type
         self._file_hash: Optional[str] = None
 
-        # TODO: select formatter automatically...?
-
     @property
     def file_hash(self) -> str:
         if self._file_hash is None:
diff --git a/PyTeX/format/auto_format.py b/PyTeX/format/auto_format.py
index a019fb8..522254d 100644
--- a/PyTeX/format/auto_format.py
+++ b/PyTeX/format/auto_format.py
@@ -9,8 +9,7 @@ from .dtx_formatter import DTXFormatter
 from .pytex_formatter import PyTeXFormatter
 
 
-def from_file_extension(
-        cls,
+def formatter_from_file_extension(
         input_file: Path,
         config: Optional[FormattingConfig] = None,
         locate_file_config: bool = True,
@@ -22,6 +21,7 @@ def from_file_extension(
         '.cls.pytex': SimpleTeXFormatter,
         '.dict.pytex': DictFormatter
     }
+    #  TODO: other formatters
     try:
         [name, extension] = input_file.name.split('.', maxsplit=1)
     except: