What I did was first convert the sentence to lower case, then declared an alphabet string to loop through it and search for each character in the sentence.

Since we're looking for pangrams that have ALL the letters in the alphabet, if the "char not in sentence_lower" is ever found true, then abort the function by returning False. If the loop completes without executing the conditional block, then return True.

More than one way to skin a cat...

class Solution:
  def checkIfPangram(self, sentence):
    sentence_lower = sentence.lower()
    alphabet = "abcdefghijklmnopqrstuvwxyz"
    for char in alphabet:
      if char not in sentence_lower:
        return False
    return True