Nested for loop into a list comprehension

list_comprehension_nested_for_loop.py

substrings_list = ["an", "the"]
strings_list = ["an apple", "the tree", "all trees", "an apple in the tree"]

# For loop to create a list of (substring, string) tuples from two lists.
# First is a list substrings and the second is a list of strings to be parsed.
tuple_list = []
for string in strings_list:
  for substring in substrings_list:
    if substring in string:
      tuple_list.append((substring, string))

# Rewrite using list comprehension.
# Syntax: 
#   list_object = [expression_on_items for_item_in_iterable_1
#                  for item_in_iterable_2 if_condition_on_items]
tuple_list = [(substring, string) for string in strings_list
              for substring in substrings_list if substring in string]

print(tuple_list) 
# Output: [('an', 'an apple'), ('the', 'the tree'), 
#          ('an', 'an apple in the tree'), ('the', 'an apple in the tree')]