Identifiers in Python: Naming Rules & Best Practices

Identifiers in Python are names used to identify variables, functions, classes, modules, and other objects. Python has specific naming rules that must be followed to ensure proper functionality, such as starting with a letter or underscore, followed by letters, numbers, or underscores.

In this blog, we will explore these naming rules in detail, along with best practices for creating clear, readable, and maintainable identifiers in Python. We’ll also highlight common mistakes to avoid when naming your Python identifiers.

What are Identifiers in Python?

Different programming elements need to be identified and named uniquely to differentiate them from others of the same kind. These are referred to as Identifiers. The user defines these names according to their choice and requirements, including names for classes, functions, variables, methods, and more.

To understand what is an identifier in Python,  follow similar principles but also adhere to specific naming conventions unique to the language. Understanding what are identifiers in Python is essential for writing clean and efficient code.

These must begin with a letter (A-Z or a-z) or an underscore (_) and can be followed by letters, digits (0-9), or underscores. However, they cannot be Python keywords or contain special characters like @, #, or $.

The importance of Python identifiers goes beyond just naming conventions; it’s about how they structure your code and help manage different components of your program. Here are three programs that can help you:

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Reserved words in Python, known as keywords, cannot be used as identifiers for functions or variables. They define Python's syntax and structure. As of Python 3.7, there are 33 keywords, a number that may change over time. Except for ‘True’, ‘False’, and ‘None’, all keywords are lowercase and must be used with case sensitivity in mind.

Here's an example to showcase how you can use identifiers in Python:

# Defining identifiers
student_name = "Ravi Kumar"
student_age = 21
student_city = "Delhi"

# Using identifiers
print("Student Name:", student_name)
print("Student Age:", student_age)
print("Student City:", student_city)

Output:

Student Name: Ravi Kumar

Student Age: 21

Student City: Delhi

Explanation:

• student_name, student_age, and student_city are all identifiers in Python. They represent different pieces of information about the student.
• These Python identifiers are valid because they follow the language’s naming rules: they start with a letter and contain only letters, digits, or underscores.
• The variables are used in the print() statements to output the values associated with them.

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Now that we understand what identifiers are in Python, let's explore the rules that govern how they should be named.

Python Identifier Naming Rules

When working with Python identifiers, it's crucial to follow the Python identifier rules to ensure your code runs smoothly and is easy to understand. These rules exist to prevent errors and maintain clarity, making it easier for developers to read and maintain code. 

For example, a rule like "identifiers must not start with a number" helps avoid ambiguity. If 1var were allowed as an identifier, it could confuse the interpreter and make the code harder to debug. 

Following Python identifier rules ensures your code is both functional and readable.

• Identifier names in Python can contain numbers (0-9), uppercase letters (A-Z), lowercase letters (a-z), and underscore (_).  
• The name should always start with a non-numeric character.
• An identifier name should not contain numeric characters only.
• Identifier names in Python are case-sensitive like most other languages. (‘Ash’ is different from ‘ASH’).
• Users can begin identifiers with an underscore; it will not display an error.
• An identifier name can be of any length, although the PEP-8 standard rule advises limiting the number of characters in a line to 79.
• Identifiers should not carry the same name as keywords. (To avoid errors in this regard, users can type help() followed by “keywords” to view a list of all the keywords in Python.)
• Leading double underscores (__$) cannot be used as these category names are used for the context of the definition of a class. These are private variables of derived and base classes. 

Deviation from any of these Python identifier rules  mentioned above may lead to error reports and cause runtime issues in the program.

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Having covered the naming rules for Python identifiers, let's now look at some examples to distinguish between valid and invalid identifiers.

Valid vs Invalid Python Identifiers (With Examples)

When working with identifiers in Python, it's important to understand the difference between valid and invalid identifiers in Python. A valid identifier in Python follows the rules and conventions set by the language, while an invalid identifier in Python breaks those rules. 

Now let’s explore examples of valid and invalid identifiers in Python in the table below.

Valid Identifiers:

# Valid Python identifiers
student_name = "Aarav Sharma"
user_age = 22
total_sales = 25000
_employee_id = "EMP1234"

# Using valid identifiers
print("Student Name:", student_name)
print("User Age:", user_age)
print("Total Sales:", total_sales)
print("Employee ID:", _employee_id)

Output:

Student Name: Aarav Sharma

User Age: 22

Total Sales: 25000

Employee ID: EMP123450

Explanation:

This code shows valid Python identifiers. Four variables are defined: student_name, user_age, total_sales, and _employee_id, each holding respective data. The print() function then displays the values of these variables. The identifiers follow Python's naming rules and store information about a student, their age, sales, and an employee ID.

Invalid Identifiers:

# Invalid Python identifiers
123var = 15  # Error: Starts with a digit
@username = "Rajesh"  # Error: Contains special character '@'
if = "Test"  # Error: 'if' is a reserved keyword
total-sales = 7000  # Error: Contains a hyphen '-'
user name = "Neha"  # Error: Contains a space

Explanation:

This code demonstrates invalid Python identifiers. Variables like 123var, @username, if, total-sales, and user name are invalid due to issues like starting with a digit, containing special characters, using reserved keywords, having a hyphen, or including spaces. These violate Python's naming rules and will result in errors.

Explanation of the Difference:

• Valid Identifiers:
  • student_name, user_age, and total_sales are clear, readable identifiers that follow Python identifier rules. These names make the code easy to understand and maintain.
  • _employee_id is valid even though it starts with an underscore. This is a convention used to indicate that the variable is intended for internal or private use.
• Invalid Identifiers:
  • 123var is invalid because it starts with a digit. Python identifiers cannot begin with a number; they must start with a letter or an underscore.
  • @username is invalid because it contains the special character @, which is not allowed in Python identifiers.
  • if is a Python keyword, and keywords cannot be used as identifiers. Using them as identifiers will lead to a syntax error.
  • total-sales is invalid because it contains a hyphen (-). Python identifiers can only contain letters, digits, and underscores.
  • user name is invalid because it contains a space. Spaces are not allowed in Python identifiers; instead, underscores should be used to separate words (e.g., user_name).

Understanding valid and invalid identifiers in Python is essential for writing error-free Python code. Knowing what are identifiers in Python and following Python identifier rules will help you avoid common mistakes and follow best practices.

Also Read: Types of Data Structures in Python: List, Tuple, Sets & Dictionary

Now that we've seen examples of valid and invalid Python identifiers, let's explore how you can check the validity of an identifier in Python.

How Check Identifier Validity in Python? 

Python has a function which developers can use to check if an identifier name will be declared valid or not. It is the function identifier().

However, the limitation of this function is that it does not consider reserved keywords for identification.

To overcome this limitation, Python provides another function known as keyword identifier(). This function checks the validity of an identifier name while keeping the keywords in mind.

For example; 

print(“xyz”.isidentifier()) 

print(“88x”.isidentifier()) 

print(“_”.isidentifier())

print(“while”.isidentifier())

Output:

True

False

True

True (incorrect output)

There is another function str.isidentifier(), that can determine whether an identifier name is valid or not. 

Now that you understand the validity of Python identifiers, focus on applying these rules consistently in your code. Choose clear, descriptive names that align with best practices for Python identifiers.

Also Read: Java Identifiers: Definition, Syntax, & Best Practices 2025

Having learned how to check the validity of identifiers in Python, let's now focus on the best practices for naming identifiers effectively.

Best Practices for Naming Identifiers

Following Python’s rules ensures valid identifiers in Python, but using professional naming practices helps prevent subtle issues. While these errors may not cause syntax issues right away, they can lead to runtime or logical errors, producing unexpected results. 

Syntax errors occur when the code violates rules, while runtime errors happen when incorrect logic alters the program’s flow.

Here are the best practices for Python Identifiers:

1. Naming Constants

Constants should be easily distinguishable from variables, and naming them in all uppercase letters helps achieve this. It also makes constants more noticeable and easier to understand in code.

• Use all uppercase letters.
• Separate words with underscores.
• Example: MAX_VALUE, SUMMATION_INDEX.

2. Naming Package Names

Package names should be short, simple, and clear. Using lowercase letters without underscores ensures consistency and better readability when importing packages.

• Keep names short and descriptive.
• Avoid underscores.
• Use all lowercase letters.
• Example: utilities, math.

3. Naming Class Names

Class names should begin with uppercase letters to differentiate them from other identifiers. For multi-word names, capitalize the first letter of each word to make the name more readable and structured.

• Start with an uppercase letter.
• Capitalize the first letter of each word for multi-word names.
• Example: Matrix, BubbleSort, ElectricBill.

Apart from this following are the best naming practices for identifiers in Python:

• If the identifier consists of two underscores, one at the beginning and one at the end, the identifier name is a language-defined special. Users should avoid this technique of naming.
• Generally, names of functions that return Boolean values begin with ‘is’. For example, isstring, iskeyword, etc.
• Identifier names can be of any length. But one should keep it short and precise for efficient usage. Like, First_inputed_value is acceptable, but it is better to use InpValue_1
• Identifier names should be kept meaningful for a better understanding of the program. To provide examples, ‘HRAvalue: conveys the underlying message better than ‘Asdf0022’.
• Technically, one can use underscores as the first and last characters, but it is advised not to do so because that format is used for Python built-in types.
• If the names of variables models for functions contain more than one word, then it is better to separate them with an underscore. Example: is_true(), Input_array(), object_inputted, etc. 
• Generally, module functions and variable names begin with lowercase alphabets. For example: dataentry(), pattern_1, etc. 

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Now that we've covered the best practices for naming identifiers, let's look at some common mistakes to avoid when working with them in Python.

Common Mistakes to Avoid When Using Identifiers in Python

When working with Python identifiers, it's easy to make simple mistakes that can cause your code to fail or become hard to read. Here, we’ll highlight some of the most common mistakes to avoid, ensuring your code stays clean and error-free.

1. Using Python Keywords as Identifiers
   Python identifiers cannot be any of the reserved keywords like for, if, else, or class. Using them as identifiers in Python will result in a syntax error.
2. Starting Identifiers with Numbers
   Python identifiers must begin with a letter or an underscore, not a number. Using a number as the first character will cause the identifier to be invalid.
3. Using Special Characters
   Avoid using special characters like @, #, -, or spaces in your Python identifiers. These characters aren’t allowed and will make the identifier invalid.
4. Overusing Underscores
   While underscores are allowed, overusing them (e.g., my___var) can make your Python identifier harder to read. Keep it simple and clear.
5. Not Following Naming Conventions
   Not using clear, descriptive names for Python identifiers can make your code harder to understand. Stick to best practices for Python identifiers like using lower case with underscores for variables and upper case for constants.

Avoiding these common mistakes will help you use Python identifiers effectively and make your code more readable and less error-prone.

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After reviewing the common mistakes to avoid, it’s important to understand the best practices for naming identifiers in Python. Learn how to do this effectively with upGrad.

Learn Best Practices for Naming Identifiers in Python with upGrad

Identifiers in Python follow specific naming rules that help ensure code clarity and prevent errors. By adhering to best practices, such as using uppercase for constants and following appropriate naming conventions for variables, functions, and classes, you can improve the readability and maintainability of your code.

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