Improve password generation

Passwords were generated by randomly choosing items in a single group formed by
the required characters, digits and symbols. The problem with that way
of choosing items is that it doesn't consider the types of the items,
thus getting more chances of choosing a letter than a number, given that
the letters group is bigger than the numbers group.
Now, I create a list with each required type of element (lower case
chars, upper case chars, digits and symbols) and randomly pick one. Then
I pick an item of that group. This way, each type of element has the
same probability of being picked.

passphrase/passphrase.py

@@ -210,26 +210,26 @@ def _read_words_from_diceware(inputfile: str) -> list:
             word.split()[1] for word in open(inputfile, mode='rt')
         ]
 
-    def _get_password_characters(self) -> str:
+    def _get_password_characters(self, cathegorized=False) -> str:
         from string import (
             digits,
             ascii_lowercase,
             ascii_uppercase,
             punctuation
         )
 
-        characters = ''
+        group = []
 
         if self.password_use_lowercase:
-            characters += ascii_lowercase
+            group.append(ascii_lowercase)
         if self.password_use_uppercase:
-            characters += ascii_uppercase
+            group.append(ascii_uppercase)
         if self.password_use_digits:
-            characters += digits
+            group.append(digits)
         if self.password_use_punctuation:
-            characters += punctuation
+            group.append(punctuation)
 
-        return characters
+        return group if cathegorized else ''.join(group)
 
     def __init__(
             self,
@@ -472,16 +472,18 @@ def generate_password(self) -> list:
         """Generates a list of random characters."""
 
         password = []
-        characters = self._get_password_characters()
+        characterset = self._get_password_characters(cathegorized=True)
         if (
                 self.passwordlen is None
-                or not characters
+                or not characterset
         ):
             raise ValueError("Can't generate password: character set is "
                              "empty or passwordlen isn't set")
 
         for _ in range(0, self.passwordlen):
-            password.append(randchoice(characters))
+            # The first choice is done to choose the set
+            # The second chooses an item in that set
+            password.append(randchoice(randchoice(characterset)))
 
         self.last_result = password
         return password