Identifiers In Python | Rules, Validity, & Best Practices (+Examples)

Have you ever tried to name a file on your computer and received an error because you used a weird character or started the name with a number? Programming languages are picky like that, too. This means that you cannot give code components any name you desire. And this is where the concept of identifiers in Python comes in. But what are they?

In Python, just like in any other language, we need to name things: variables, functions, classes, user-defined modules–you name it (literally). These names are called identifiers. In this article, we will discuss everything you need to know about identifiers in Python–from what they are and how they work to naming rules, best practices, and the difference between identifiers and Python's reserved keywords. By the end, you'll not only understand them but also know how to avoid any common pitfalls.

What Are Identifiers In Python?

In Python, an identifier is simply the name you use to identify a variable, function, class, module, or object. It acts as a reference to a value stored in memory. Whenever you write something like score = 100 or def greet():, you're using an identifier (score, greet) to name something meaningful (here, variable and function, respectively) in your program.

Think of identifiers as the labels you put on folders in your computer. Instead of trying to remember what’s inside every folder based on its creation date or size, you just label it "Photos", "CollegeDocs", or "Taxes2025". Similarly, in code, identifiers make your programs readable and logical.

Real-life Analogy:

Imagine a classroom. Each student has a name, right? That name helps the teacher take attendance, assign grades, and interact with them. But you can't have two students with the exact same name sitting next to each other and expect no confusion. Similarly, Python uses identifiers to keep track of different "things" in your program–clearly and unambiguously.

In summary:

• Identifiers are names given to Python objects.
• They allow you to assign, access, and manipulate data.
• Every variable, function you define, class, etc., needs an identifier to be referenced.

Rules For Naming Identifiers In Python

Python has a clean and readable syntax–but that also means it expects you to name things properly. Identifiers (names for variables, functions, classes, etc.) must follow a strict set of rules. Break them, and Python will raise an error even before your code runs. Let’s walk through each rule with an explanation and snippet examples.

Rule 1: Identifiers can contain letters (A–Z or a–z), digits (0–9), and underscores (_)

These are the only characters allowed in a Python identifier. Special symbols like @, #, !, -, or spaces are not allowed because they have other meanings in Python syntax. For example, the symbol (-) is the subtraction arithmetic operator, and @ is a decorator symbol.

Why do we do this? Because restricting the character set helps Python parse code unambiguously. For example:

name        # ✅ valid
user_123    # ✅ valid

Rule 2: Identifiers must begin with a letter or an underscore (_) and not a digit

Python doesn’t allow identifiers to start with a number. That’s because this can lead to the identifiers being confused with numeric literals.

Why? If 1value = 50 were valid, Python might confuse it with a numeric constant or expression.

_name       # ✅ valid
age1        # ✅ valid
1st_value   # ❌ invalid — starts with a digit

Rule 3: Identifiers are case-sensitive

In Python, when we case identifiers are case sensitive, it means that– Name, name, and NAME are three completely different identifiers. This can be a source of subtle bugs if you're not careful, especially in larger projects or while collaborating with others.

Why? Case sensitivity gives you more flexibility but requires consistency and caution. Example:

price = 100
Price = 200
# These are two separate variables!

Rule 4: You cannot use Python keywords as identifiers

Python reserves certain words for its own syntax–like if, else, class, return, etc. You cannot use these as names for defining variables, functions, or anything else.

Why?: Keywords have predefined meanings in Python. Using them would confuse the interpreter. Example:

class = "Math"     # ❌ invalid — 'class' is a reserved keyword
def = 25           # ❌ invalid — 'def' is used for defining functions

We will discuss keywords and how they differ from identifiers later. But if you want to check all the keywords in the language, you can do so by importing the keyword module and then calling the built-in function .kwlist(), as follows:

import keyword
print(keyword.kwlist)

Rule 5: Identifiers cannot contain special characters like @, $, %, -, or spaces

Special characters serve different roles in Python syntax and cannot be part of identifier names. For instance, the minus sign (-) is used for subtraction operations, and the rate symbol (@) is used for decorators.

Why do we do this? Including these would make it hard to distinguish between identifiers and operators or symbols. Example:

user-name    # ❌ invalid — contains a hyphen
email@id     # ❌ invalid — contains '@'
first name   # ❌ invalid — contains space

Rule 6: There’s no fixed length limit, but don’t go overboard

Technically, you could name a variable totalAmountUserPaidOnApril21stBeforeLunch, and Python wouldn’t complain. But long names become hard to read and maintain. It is, hence, important to strike a balance between clarity and brevity.

Pro Tip: Good names should describe what the variable stores, not the whole story. Example:

x = 100                      # ✅ valid but vague
total_amount = 100          # ✅ valid and descriptive
totalAmountUserPaid...      # ✅ but too long to be readable

Key Takeaways for Naming Indentifers in Python

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Valid & Invalid Identifiers In Python

Once you know the rules, it's useful to see how they apply in real examples. Identifiers are everywhere in Python code, so understanding what makes them valid or invalid is essential to avoiding frustrating syntax errors. In this section, we will explore what are valid and invalid identifiers in Python language.

What Makes an Identifier Valid?

Based on the rules of naming identifiers, an identifier is valid if:

• It follows Python’s naming rules (starts with a letter or _, contains only allowed characters, and doesn’t use a keyword).
• It is unique and meaningful in context.

What Makes an Identifier Invalid?

In contrast, an identifier in Python programs is invalid if:

• It starts with a digit.
• It uses disallowed characters like @, -, or whitespace.
• It matches a Python keyword like if, class, def, etc.

Code Example:

Output:

 File "main.py", line 1
    1st_place = "Gold"
    ^
SyntaxError: invalid decimal literal

Explanation:

• We create a variable, but when Python sees 1st_place starting with a digit (1), it thinks you're trying to write a number.
• The letters st_place that follow confuse the interpreter, resulting in a SyntaxError as shown in the output.

Examples of Valid and Invalid Identifiers in Python

How To Test Validity Of Identifiers In Python

Sometimes, especially in dynamic programming or while reading/taking user input, you may want to check whether a string is a valid identifier before using it in your code. Python language provides a simple built-in method to do this.

The str.isidentifier() Method to Check Indentifiers in Python

Python strings have a built-in method called .isidentifier() that returns True if the string is a valid identifier and False otherwise. You can use this to check if the identifier is valid or not.

Note: This method only checks for syntactic validity. It does not check whether the identifier is a keyword.

Code Example:

Output:

True
False
True
True

Explanation:

• We use the string method .isidentifier() on different string values, and display the outcome using the print() function. 
• In the first case, the "name".isidentifier() returns True because it's a perfectly valid identifier. Same for the “user_name”.isidentifier() call.
• But, "123abc" fails because identifiers can't start with digits.
• For the last case, the “class”.identifier() returns True since the identifier is valid. But the function ignores that “class” is actually a keyword.

So what to do if you also want to check if the identifier you are using is a keyword or not?

Checking for Keywords, Too?

If you want to check whether something is both a valid identifier and not a keyword, use a combination of the isidentifer() method with the iskeyword() method from the Python library.

Code Example:

Output:

False

Explanation:

Here we first import the keyword module and then create an identifier variable with the string value “class”.

• Then, we check if this is valid by combining the .isidentifier() and .iskeyword() methods using the and not logical operators.
• Here, "class" passes .isidentifier() but is still not usable because it’s a keyword, and the outcome is False. That’s why you pair it with the keyword.iskeyword() method when needed.

When is this useful?

• Validating dynamically generated variable names.
• Input sanitization in code editors, compilers, or interpreters.
• Writing Python tools or linters.

Best Practices For Using Identifiers In Python

Just because an identifier is syntactically valid doesn’t mean it’s a good choice. Python emphasizes code readability, and good naming is a big part of that. The PEP 8 style guide by Python offers conventions that help you write Pythonic code. Here are some tried-and-true best practices to follow:

1. Use Descriptive Names

Your identifiers should reflect the purpose of the variable, function, or class in Python programs. As a rule of thumb, avoid single-letter names unless used in very short scopes like loop counters. For example:

x = 500 # Bad
monthly_salary = 500 # Good

Why? Code is read more often than it's written. Names should make the code self-explanatory.

2. Use snake_case for Variables and Functions

Python recommends using lowercase letters with underscores for naming variables and functions. For example:

user_score = 85 # Good
def calculate_average(): … # Good

Avoid using camelCase like userScore for variables–it’s common in other languages like JavaScript but not Pythonic.

3. Use PascalCase for Class Names

Classes should follow the PascalCase convention (capitalizing each word and omitting underscores). For example:

class StudentRecord:
    pass

4. Use ALL_CAPS for Constants

When defining a constant (a variable that’s not supposed to change), it is advisable to use all capital letters and underscores. For example:

PI = 3.14159
MAX_USERS = 1000

5. Prefix Internal/Private Names with an Underscore

In Python, a single leading underscore is a hint that a variable or method is intended for internal use. So try to avoid using them when creating your own identifiers. For example:

def _helper_function(): …
_private_value = 42

This doesn’t enforce privacy (like in Java or C++), but signals intent to other developers.

6. Avoid Using Python Keywords or Built-ins

Even though str, list, id, etc., can technically be overwritten as identifiers, you shouldn’t do it. The snippet example below highlights what you should avoid:

list = [1, 2, 3]      # Overwrites built-in 'list'
id = 101              # Overwrites built-in 'id'

Why?: This can break built-in functionality and lead to confusing bugs.

7. Keep It Short but Meaningful

Avoid overly long names, but don’t sacrifice clarity. If you find yourself typing a name that's too long, consider breaking it into helper variables or functions. For example:

total_revenue = income + bonus # Acceptable
total_revenue_generated_by_employee_during_fiscal_year = … # Too much

8. Be Consistent

Use a consistent naming convention across your project or team. If you use snake_case for one function, use it for all.

What Are Keywords In Python?

A keyword in Python is a word that is reserved by the language and used to define the structure and syntax of the language. They represent the basic building blocks of the language (e.g., if, else, class, while).

In short, keywords are reserved words that have a special meaning to the interpreter. They cannot be used as identifiers (variable names, function names, class names, etc.) because they are already predefined to perform specific functions in the language.

Why shouldn’t we use keywords as identifiers? If Python allowed you to use keywords as identifiers, it would confuse the interpreter and break the language’s syntax.

To know more about the set of keywords in the Python programming language, read: Python Keywords | All 39 Reserved & Soft Explained +Code Examples

Pro Tip: It might be a good idea to check for keywords when confused about using one as an identifier. As mentioned before, you can easily get a list of all keywords in Python using the keyword module as follows:

import keyword
print(keyword.kwlist)

This will output a list of all current keywords in Python, which may change slightly with newer versions. By understanding Python keywords, you can avoid mistakes in naming your identifiers and can appreciate how they define the flow and structure of your Python programs.

Difference Between Keywords & Identifiers In Python

Understanding the difference between keywords and identifiers is essential for mastering Python syntax. Both play crucial roles in your Python code, but they serve very different purposes.

What Are Keywords?

As we’ve just discussed, keywords are reserved words in Python that are predefined to serve specific functions. They are part of the language's core structure and cannot be used as identifiers. Keywords form the building blocks of Python syntax.

What Are Identifiers?

On the other hand, identifiers are names you choose for variables, functions, classes, and other entities in your code. They are how you reference data or behavior in your programs. Identifiers must follow certain rules (like not starting with a digit), but they are not reserved by Python and can be named according to your needs.

Key Differences Between Keywords & Identifiers In Python

Why the Distinction Matters?

• Keywords are part of Python’s core language syntax and structure. You need to respect them, and Python will throw an error if you try to use them as identifiers.
• Identifiers, on the other hand, are chosen by you to represent specific values or behavior. While they are free-form, following best practices makes your code more readable and maintainable.

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Conclusion

In this article, we’ve explored the concept of identifiers in Python, breaking down their rules, proper naming conventions, and how they differ from keywords. We also learned how to test the validity of identifiers and adopted best practices for writing clean, readable code.

• Identifiers are names you use for variables, functions, classes, and other entities, and they must adhere to Python’s naming rules.
• Keywords are reserved words that have predefined meanings in Python, and they cannot be used as identifiers.
• Following the rules and best practices for naming identifiers will help you write Python code that’s easy to read, maintain, and debug.

As you continue to write Python code, being mindful of these rules will help you avoid errors and make your code more Pythonic.

Frequently Asked Questions

Q1. Can I use a keyword as an identifier in Python?

No, Python will not allow you to use keywords (like if, while, class, etc.) as identifiers. Keywords are reserved by the language for specific purposes, and trying to use them as variable names will result in a syntax error.

Q2. What happens if I use an invalid identifier in my code?

If you use an invalid identifier (e.g., one starting with a number, containing spaces, or using special characters like @), Python will raise a SyntaxError and stop executing the code.

Q3. Are identifiers case-sensitive in Python?

Yes, Python is case-sensitive. This means that the names/identifiers– total and Total would be treated as two distinct identifiers. This is why you must always be consistent with your capitalization and avoid confusion.

Q4. Can I use underscores in Python identifiers?

Yes, underscores are allowed in Python identifiers. They are commonly used in variable names to separate words (e.g., user_age). However, an identifier starting with an underscore (_variable) is often used to signal that it’s intended for internal use (though this is not enforced by Python).

Q5. Can I use Python built-in names like list or str as identifiers?

Technically, you can assign values to built-in names like list or str, but it’s strongly discouraged. Overwriting these built-in names can lead to bugs and unexpected behavior in your program since you’ll lose access to the original functionality of these built-ins.

Q6. How do I check if a name is a valid identifier in Python?

You can use the .isidentifier() method to check if a string is a valid identifier in Python. However, note that this method only checks the syntax and not whether the identifier is a keyword.

Example:

Here, we import the keyword module and create a variable identifier with the string value “class”. Then, we use an if-else statement to verify if the identifier is valid and not a keyword. If the condition is met the outcome is “Valid identifier” and is not, then the else block prints “Invalid Identifier”.

Q7. Can I use an identifier as a variable name inside a function?

Yes, you can use identifiers inside functions. However, be mindful of the scope, i.e., identifiers defined inside a function are local to that function, and you cannot access them outside of it unless returned or passed explicitly.

This compiles our discussion on identifiers in Python. Do check the following out:

• Python List | Everything You Need To Know (With Detailed Examples)
• Python String Manipulation | Handbook With Techniques & Examples
• Keyword Arguments In Python: What, Why & How To Use (+Examples)
• Hello, World! Program In Python | 7 Easy Methods (With Examples)
• Relational Operators In Python | All 6 Types With Code Examples