Move list validation to own function

Lists are now checked via plotoptions.validate_list(). This function iterates through the list and checks if all elements are of the given type (default: str). Optionally the strings can be checked if they correspond to an existing file.
Closes #64 (closed)

plotid/plotoptions.py

@@ -2,6 +2,8 @@
 # -*- coding: utf-8 -*-
 """Contains the PlotOptions and PlotIDTransfer classes."""
 
+import os
+
 
 class PlotOptions:
     """
@@ -10,7 +12,10 @@ class PlotOptions:
     Methods
     -------
     __init__
-    validate_input : Check if input is correct type.
+    validate_input
+        Check if input is correct type.
+    validate_list
+        Check if all elements of a given list are of certain type.
 
     Attributes
     ----------
@@ -99,7 +104,55 @@ class PlotIDTransfer:
     def __init__(self, figs, figure_ids):
         self.figs = figs
         self.figure_ids = figure_ids
+        self.figure_ids = validate_list(self.figure_ids)
 
     def __str__(self):
         """Representation if an object of this class is printed."""
         return str(self.__class__) + ": " + str(self.__dict__)
+
+
+def validate_list(list_var, elt_type=str, is_file=False):
+    """
+    Validate if contents of a list are of specific type.
+
+    Parameters
+    ----------
+    list_var : list or str
+        List or single string which contents will be validated.
+    elt_type : datatype, optional
+        Datatype of which the list elements must be type of. Otherwise
+        an Error will be raised. The default is str.
+    is_file : boolean, optional
+        Flag to indicate if the list contains paths to files. If True the
+        strings will be checked if they correspond to an existing file.
+        The default is False.
+
+    Raises
+    ------
+    TypeError
+        If one of the list elements is not of type elt_type.
+    FileNotFoundError
+        If strings are also checked for existing files and one of the files
+        does not exist.
+
+    Returns
+    -------
+    list_var as list
+
+    """
+    if isinstance(list_var, elt_type):
+        list_var = [list_var]
+    if isinstance(list_var, list):
+        for elt in list_var:
+            if not isinstance(elt, elt_type):
+                raise TypeError(f'The list of {list_var} contains an '
+                                f'object which is not of type {elt_type}.')
+            if is_file:
+                # Check if directory and files exist
+                if not os.path.exists(elt):
+                    raise FileNotFoundError('The specified directory'
+                                            f'/file {elt} does not exist.')
+    else:
+        raise TypeError(f'The specified {list_var} are neither a '
+                        f'{elt_type} nor a list of {elt_type}.')
+    return list_var