Solution to problem 4 in Python

src/P4.py

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+# --- Day 4: Passport Processing ---
+
+# You arrive at the airport only to realize that you grabbed your North Pole
+# Credentials instead of your passport. While these documents are extremely
+# similar, North Pole Credentials aren't issued by a country and therefore
+# aren't actually valid documentation for travel in most of the world.
+
+# It seems like you're not the only one having problems, though; a very long
+# line has formed for the automatic passport scanners, and the delay could
+# upset your travel itinerary.
+
+# Due to some questionable network security, you realize you might be able to
+# solve both of these problems at the same time.
+
+# The automatic passport scanners are slow because they're having trouble
+# detecting which passports have all required fields. The expected fields are
+# as follows:
+
+#     byr (Birth Year)
+#     iyr (Issue Year)
+#     eyr (Expiration Year)
+#     hgt (Height)
+#     hcl (Hair Color)
+#     ecl (Eye Color)
+#     pid (Passport ID)
+#     cid (Country ID)
+
+# Passport data is validated in batch files (your puzzle input). Each passport
+# is represented as a sequence of key:value pairs separated by spaces or
+# newlines. Passports are separated by blank lines.
+
+# Here is an example batch file containing four passports:
+
+# ecl:gry pid:860033327 eyr:2020 hcl:#fffffd
+# byr:1937 iyr:2017 cid:147 hgt:183cm
+
+# iyr:2013 ecl:amb cid:350 eyr:2023 pid:028048884
+# hcl:#cfa07d byr:1929
+
+# hcl:#ae17e1 iyr:2013
+# eyr:2024
+# ecl:brn pid:760753108 byr:1931
+# hgt:179cm
+
+# hcl:#cfa07d eyr:2025 pid:166559648
+# iyr:2011 ecl:brn hgt:59in
+
+# The first passport is valid - all eight fields are present. The second
+# passport is invalid - it is missing hgt (the Height field).
+
+# The third passport is interesting; the only missing field is cid, so it looks
+# like data from North Pole Credentials, not a passport at all! Surely, nobody
+# would mind if you made the system temporarily ignore missing cid fields.
+# Treat this "passport" as valid.
+
+# The fourth passport is missing two fields, cid and byr. Missing cid is fine,
+# but missing any other field is not, so this passport is invalid.
+
+# According to the above rules, your improved system would report 2 valid
+# passports.
+
+# Count the number of valid passports - those that have all required fields.
+# Treat cid as optional. In your batch file, how many passports are valid?
+
+
+with open("files/P4.txt", "r") as f:
+    passports = [line.split() for line in f.read().strip().split("\n\n")]
+
+
+def part_1() -> None:
+    valid_passports = 0
+    for passport in passports:
+        if len(passport) > 7:
+            valid_passports += 1
+        if len(passport) == 7:
+            valid_fields = 0
+            for field in passport:
+                if "cid" not in field:
+                    valid_fields += 1
+            if valid_fields == 7:
+                valid_passports += 1
+
+    print(f"There are {valid_passports} valid passports")
+
+
+if __name__ == "__main__":
+    part_1()