Using dictionary to count the frequency of letters in the input string

Python/count_letter.py

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+def count_letters(text):
+  result = {}
+  # Go through each letter in the text
+  count  = 0
+  for letter in text.lower():
+    # Check if the letter needs to be counted or not
+    if letter.isalpha():
+      if letter not in result:
+        result[letter]=0
+      result[letter]+=1 
+    # Add or increment the value in the dictionary
+
+  return result
+
+print(count_letters("AaBbCc"))
+# output {'a': 2, 'b': 2, 'c': 2}
+
+print(count_letters("Math is fun! 2+2=4"))
+# output {'m': 1, 'a': 1, 't': 1, 'h': 1, 'i': 1, 's': 1, 'f': 1, 'u': 1, 'n': 1}
+
+print(count_letters("This is a sentence."))
+# output {'t': 2, 'h': 1, 'i': 2, 's': 3, 'a': 1, 'e': 3, 'n': 2, 'c': 1}