Return errors when found or None otherwise

This commit adds a number of functions that return the error object
when an error is found, or `None` otherwise. It avoids the use of
exceptions as control flow constructs.

1 files changed, 17 insertions, 0 deletions

diff --git a/quality_control/standard_error.py b/quality_control/standard_error.py
index f1e33c4..022cc9b 100644
--- a/quality_control/standard_error.py
+++ b/quality_control/standard_error.py
@@ -1,6 +1,8 @@
 """Contain logic for checking standard error files"""
 import re
+from typing import Union
 
+from .errors import InvalidValue
 from .errors import InvalidCellValue
 
 def valid_value(val):
@@ -11,3 +13,18 @@ def valid_value(val):
         f"Invalid value '{val}'. "
         "Expected string representing a number with at least six decimal "
         "places.")
+
+def invalid_value(line_number: int, column_number: int, val: str) -> Union[
+        InvalidValue, None]:
+    """
+    Returns a `quality_control.errors.InvalidValue` object in the case where
+    `val` is not a valid input for standard error files, otherwise, it returns
+    `None`.
+    """
+    if re.search(r"^[0-9]+\.[0-9]{6,}$", val):
+        return None
+    return InvalidValue(
+        line_number, column_number, val, (
+            f"Invalid value '{val}'. "
+            "Expected string representing a number with at least six decimal "
+            "places."))