Python Keywords and Identifiers

Every name in a Python program — variables, functions, classes, modules — is an identifier, and some words are reserved by the language itself as keywords. Knowing the rules for valid names, the list of keywords, and the naming conventions the Python community follows makes your code correct and instantly readable to other developers.

Keywords

Keywords have special meaning and cannot be used as names: False None True and as assert async await break class continue def del elif else except finally for from global if import in is lambda nonlocal not or pass raise return try while with yield. Python also has soft keywords — match, case, type and _ — that are keywords only in specific contexts, so they can still be used as ordinary names elsewhere. The keyword module lists them all.

Identifier Rules

A valid identifier:

• Starts with a letter (a–z, A–Z, or Unicode letters) or an underscore _.
• Continues with letters, digits or underscores — no spaces, hyphens or symbols like @ $ %.
• Cannot start with a digit (2nd_place is invalid).
• Cannot be a keyword (class, for...).
• Is case-sensitive: total, Total and TOTAL are three different names.

Naming Conventions (PEP 8)

Beyond validity, follow PEP 8 so code looks familiar: snake_case for variables, functions and modules; PascalCase for classes; UPPER_SNAKE_CASE for constants; a leading underscore (_internal) for "private" names; and double leading underscores (__secret) trigger name mangling inside classes. Avoid shadowing built-ins such as list, str, id or sum.

Examples

import keyword

print("Number of keywords:", len(keyword.kwlist))
print("Soft keywords:", keyword.softkwlist)
print(keyword.iskeyword("for"), keyword.iskeyword("match"))

for name in ["total_marks", "_count", "2nd_place", "first-name", "class", "Café"]:
    valid = name.isidentifier() and not keyword.iskeyword(name)
    print(f"{name!r:15} valid identifier: {valid}")

Number of keywords: 35
Soft keywords: ['_', 'case', 'match', 'type']
True False
'total_marks'   valid identifier: True
'_count'        valid identifier: True
'2nd_place'     valid identifier: False
'first-name'    valid identifier: False
'class'         valid identifier: False
'Café'          valid identifier: True

MAX_STUDENTS = 30                     # constant


class CourseEnrollment:              # class: PascalCase
    def __init__(self, course_name):
        self.course_name = course_name
        self._students = []           # "internal" by convention

    def add_student(self, student_name):   # method: snake_case
        if len(self._students) < MAX_STUDENTS:
            self._students.append(student_name)


enrollment = CourseEnrollment("Python")
enrollment.add_student("Asha")
print(enrollment.course_name, enrollment._students)

Python ['Asha']

score = 10
Score = 20
print(score, Score)

list = [3, 1, 2]          # shadows the built-in list() — avoid this!
try:
    print(list("abc"))
except TypeError as error:
    print("TypeError:", error)

10 20
TypeError: 'list' object is not callable