Refactor the to_string function to make it more readable

Author: seisman

pygmt/alias.py

@@ -84,27 +84,25 @@ def to_string(
     >>> to_string("invalid", mapping={"mean": "a", "mad": "d", "full": "g"})
     'invalid'
     """
-    # Return None if the value is None or False.
-    if value is None or value is False:
+    if value is None or value is False:  # None and False are converted to None.
         return None
-    # Return an empty string if the value is True. We don't have to check 'prefix' since
-    # it defaults to an empty string!
-    if value is True:
+    if value is True:  # True is converted to an empty string with the optional prefix.
         return f"{prefix}"
 
-    # Convert any value to a string or a sequence of strings.
-    if is_nonstr_iter(value):  # Is a sequence.
-        value = [str(item) for item in value]  # Convert to a sequence of strings
-        if separator is None:
-            # A sequence is given but separator is not specified. Return a sequence of
-            # strings, to support repeated GMT options like '-B'. 'prefix' makes no
-            # sense and is ignored.
-            return value
-        value = separator.join(value)  # Join the sequence by the separator.
-    elif mapping:  # Mapping long-form arguments to short-form arguments.
-        value = value[0] if mapping is True else mapping.get(value, value)
-    # Return the final string with the optional prefix.
-    return f"{prefix}{value}"
+    # Convert a non-sequence value to a string.
+    if not is_nonstr_iter(value):
+        if mapping:  # Mapping long-form arguments to short-form arguments.
+            value = value[0] if mapping is True else mapping.get(value, value)
+        return f"{prefix}{value}"
+
+    # Convert a sequence of values to a sequence of strings.
+    # In some cases, "prefix" and "mapping" are ignored. We can enable them when needed.
+    _values = [str(item) for item in value]
+    if separator is None:
+        # Sequence is given but separator is not specified. Return a sequence of strings
+        # for repeatable GMT options like '-B'.
+        return _values
+    return f"{prefix}{separator.join(_values)}"
 
 
 @dataclasses.dataclass