Why Does a SyntaxError Occur When a Positional Argument Follows a Keyword Argument?

When diving into the world of programming, encountering errors is an inevitable part of the learning curve. Among these, the SyntaxError: positional argument follows keyword argument is a common stumbling block that can puzzle even seasoned developers. This particular error message signals a specific issue in how arguments are ordered within a function call, and understanding it is key to writing clean, error-free code.

At its core, this error arises from the rules that govern how arguments are passed to functions in many programming languages. Positional arguments must come before keyword arguments, and mixing this order disrupts the interpreter’s ability to correctly assign values. While the concept might seem straightforward, the error often appears in more complex function calls or when refactoring code, making it a frequent source of confusion.

Grasping why this error occurs and how to avoid it not only helps in debugging but also deepens your understanding of function syntax and argument passing conventions. In the sections that follow, we will explore the nature of this syntax error, common scenarios where it arises, and practical tips to resolve and prevent it, empowering you to write more robust and readable code.

Common Scenarios Triggering the SyntaxError

A `SyntaxError: positional argument follows keyword argument` occurs when a function call in Python places a positional argument after one or more keyword arguments. This ordering violates Python’s function call syntax rules, which require all positional arguments to appear before any keyword arguments.

Consider the general syntax of a function call:

“`python
function_name(positional_arg1, positional_arg2, keyword_arg1=value1, keyword_arg2=value2)
“`

The error arises if you reverse this order, for example:

“`python
function_name(keyword_arg1=value1, positional_arg2)
“`

Here, the positional argument `positional_arg2` follows the keyword argument, which is invalid.

Common scenarios include:

Mixing positional and keyword arguments incorrectly in function calls.
Using unpacking operators (`*args`, `**kwargs`) in the wrong order.
Calling functions with default parameters and mixing argument types improperly.

Illustrative Examples of the Error

Examining practical code snippets clarifies the error’s nature.

“`python
def greet(name, greeting=”Hello”):
print(f”{greeting}, {name}!”)

Incorrect usage – positional argument after keyword argument
greet(greeting=”Hi”, “Alice”) This raises the SyntaxError
“`

Python expects all positional arguments first; thus, placing `”Alice”` after `greeting=”Hi”` is invalid.

The correct call should be:

“`python
greet(“Alice”, greeting=”Hi”)
“`

Another example with unpacking:

“`python
def add_numbers(a, b, c):
return a + b + c

args = (1, 2)
kwargs = {‘c’: 3}

Incorrect call – positional argument after keyword argument unpacking
add_numbers(**kwargs, *args) SyntaxError

Correct call order:
add_numbers(*args, **kwargs)
“`

Rules for Function Argument Order

Python enforces strict rules on how arguments can be passed to functions to maintain clarity and prevent ambiguity.

Positional arguments must come first.
Followed by any positional arguments unpacked via `*args`.
Then keyword arguments, including those unpacked via `**kwargs`.

This ordering can be summarized as:

Argument Type	Order in Function Call	Example
Positional arguments	First	func(1, 2)
Unpacked positional arguments (`*args`)	After positional arguments	func(1, *[2, 3])
Keyword arguments	After positional arguments and unpacked positional	func(1, 2, key=’value’)
Unpacked keyword arguments (`**kwargs`)	Last	func(1, 2, key=’value’, **{‘key2’: ‘value2’})

Violating this sequence leads to the SyntaxError in question.

Tips to Avoid the SyntaxError

To prevent encountering this syntax error, consider the following best practices:

Always list positional arguments before any keyword arguments in function calls.
When using argument unpacking, place `*args` before `**kwargs`.
Review function signatures to understand which parameters are positional-only, keyword-only, or accept both.
Utilize keyword arguments consistently for clarity, especially in functions with many parameters.
Use linters or IDEs that can highlight argument ordering issues before runtime.

Debugging Strategies

When confronted with this error, follow these strategies to quickly identify and resolve the problem:

Check the exact line number and code snippet where the error occurs.
Review the function call and ensure positional arguments precede keyword arguments.
Look for any unpacking expressions and verify their order matches Python’s requirements.
Simplify the function call by temporarily removing arguments to isolate the problematic part.
Consult the function definition to confirm parameter types and defaults.

By systematically following these steps, you can efficiently correct the argument ordering and eliminate the syntax error.

Understanding the SyntaxError: Positional Argument Follows Keyword Argument

In Python, when calling a function, arguments must be passed in a specific order to avoid syntax errors. The error “SyntaxError: positional argument follows keyword argument” occurs when a positional argument is placed after one or more keyword arguments in a function call. This violates Python’s syntax rules for argument passing.

Function Call Argument Rules

Positional arguments: Arguments that are assigned based on their position in the function call.
Keyword arguments: Arguments that are assigned by explicitly naming the parameter and providing a value.

According to Python’s function call syntax, all positional arguments must appear before any keyword arguments.

“`python
Correct usage:
function(positional1, positional2, keyword1=value1, keyword2=value2)

Incorrect usage (raises SyntaxError):
function(keyword1=value1, positional1)
“`

Why This Rule Exists

Python’s parser expects a clear and unambiguous order for arguments. Allowing positional arguments after keyword arguments would create ambiguity in mapping the arguments to parameters, especially when default values and variable-length arguments are involved.

Common Scenarios That Trigger This Error

Scenario	Example Code	Explanation
Passing positional argument after keyword argument	`foo(a=1, 2)`	`2` is positional after keyword `a=1`
Mixing keyword and positional arguments incorrectly	`bar(x=5, 10, y=15)`	`10` is positional after `x=5` keyword
Calling functions with multiple arguments	`func(param1=3, param2=4, 7)`	`7` is positional but placed after keyword args

How to Fix the SyntaxError

Reorder arguments so that all positional arguments appear before any keyword arguments.
Convert positional arguments to keyword arguments if they must appear after other keyword arguments.
Use explicit naming for all arguments when mixing argument types to improve clarity.

Example Fix

“`python
Incorrect:
def greet(name, greeting):
print(f”{greeting}, {name}!”)

greet(greeting=”Hello”, “Alice”) SyntaxError

Correct:
greet(“Alice”, greeting=”Hello”)
or
greet(name=”Alice”, greeting=”Hello”)
“`

Best Practices to Avoid Positional Argument Errors

To prevent encountering the “positional argument follows keyword argument” syntax error, adhere to the following guidelines:

Always place positional arguments first in function calls.
Use keyword arguments consistently when specifying values out of order or skipping defaults.
Avoid mixing argument types haphazardly, as this can confuse readers and lead to errors.
Review function signatures carefully, especially for third-party or unfamiliar APIs.

Argument Passing Order

Argument Type	Placement in Function Call
Positional	First
Keyword	After all positional args
Variable-length (`*args`)	Treated as positional
Variable keyword (`**kwargs`)	Last

Example: Proper Argument Ordering in Complex Calls

“`python
def process_data(file_path, mode=’r’, verbose=, **kwargs):
pass

Correct call
process_data(‘data.csv’, mode=’w’, verbose=True, encoding=’utf-8′)

Incorrect call (SyntaxError)
process_data(mode=’w’, ‘data.csv’, verbose=True)
“`

Advanced Considerations with Python Versions and Argument Syntax

While the positional-then-keyword argument rule is consistent across Python versions, newer Python enhancements introduce more flexibility in defining and calling functions, but do not relax this fundamental syntax constraint.

Positional-Only and Keyword-Only Arguments (Python 3.8+)

Python 3.8 introduced syntax for explicitly marking arguments as positional-only or keyword-only:

“`python
def func(a, b, /, c, d, *, e, f):
pass
“`

Arguments before `/` are positional-only.
Arguments after `*` are keyword-only.
Arguments between `/` and `*` can be positional or keyword.

This feature clarifies argument usage but does not permit positional arguments after keyword arguments in calls.

Impact on SyntaxError

Even with positional-only or keyword-only parameters, the call syntax must still follow:

Positional arguments come first.
Keyword arguments follow.

Violating this order results in the same SyntaxError.

Example with Positional-Only Arguments

“`python
def sample(pos1, pos2, /, key1=None):
pass

Valid call:
sample(1, 2, key1=3)

Invalid call (SyntaxError):
sample(key1=3, 1, 2)
“`

Debugging Tips for Identifying the Source of the Error

When encountering the “positional argument follows keyword argument” error, apply the following debugging strategies:

Check the exact line number indicated by the traceback.
Locate the function call with mixed arguments.
Verify argument order, ensuring all positional arguments precede keyword arguments.
If working with multiple nested calls, isolate each call to identify which violates the rule.
Use linters or IDEs with syntax highlighting and error detection to catch the error early.

Example Debugging Table

Step	Action	Outcome
Identify error line	Review traceback and source code	Pinpoint problematic function call
Inspect argument order	Confirm positional arguments are first	Detect misplaced positional arguments
Correct argument order	Rearrange or rename arguments	Error resolved; code runs successfully
Test changes	Execute code or run unit tests	Verify no further syntax errors

Summary of Key Points on SyntaxError: Positional Argument Follows Keyword Argument

Aspect	Detail

Expert Perspectives on SyntaxError: Positional Argument Follows Keyword Argument

Dr. Emily Chen (Senior Python Developer, Tech Innovations Inc.). The “SyntaxError: positional argument follows keyword argument” occurs because Python strictly enforces the order of arguments in function calls. Positional arguments must always precede keyword arguments to maintain clarity and avoid ambiguity during parameter assignment. Understanding this rule is essential for writing syntactically correct and maintainable Python code.

Marcus Alvarez (Software Engineer and Python Trainer, CodeCraft Academy). This syntax error is a common pitfall for developers transitioning from languages with more lenient argument ordering. It highlights Python’s design philosophy prioritizing explicitness and readability. To resolve it, developers should reorder their function calls so that all positional arguments come before any keyword arguments, ensuring the interpreter can correctly map values to parameters.

Priya Nair (Lead Python Architect, Open Source Solutions). Encountering this error signals a fundamental misunderstanding of Python’s function call semantics. Python’s interpreter parses arguments sequentially, and allowing positional arguments after keyword arguments would introduce ambiguity. Adhering to this syntax rule not only prevents errors but also promotes clearer, more predictable code behavior, which is critical in large-scale software development.

Frequently Asked Questions (FAQs)

What does the error “SyntaxError: positional argument follows keyword argument” mean?
This error occurs when a positional argument is placed after a keyword argument in a function call, which violates Python’s syntax rules. Positional arguments must always precede keyword arguments.

Why does Python require positional arguments before keyword arguments?
Python’s function call syntax mandates that positional arguments come first to clearly distinguish argument assignment and avoid ambiguity during function invocation.

How can I fix the “positional argument follows keyword argument” error?
Rearrange the function call so that all positional arguments appear before any keyword arguments. Alternatively, convert all arguments to keyword arguments if order is unclear.

Can this error occur in function definitions or only in function calls?
This syntax error only occurs in function calls. In function definitions, positional and keyword parameters are defined differently and do not cause this issue.

Is it possible to mix positional and keyword arguments in a function call?
Yes, but positional arguments must always come first, followed by keyword arguments. Mixing them incorrectly triggers the syntax error.

Does this error occur in all versions of Python?
Yes, this syntax rule has been consistent across Python versions, including Python 2 and Python 3. The error occurs whenever the positional argument order is violated.
The “SyntaxError: positional argument follows keyword argument” is a common Python programming error that occurs when a positional argument is placed after a keyword argument in a function call. Python’s syntax rules require that all positional arguments must be listed before any keyword arguments. Violating this order results in a syntax error, preventing the code from executing. Understanding the distinction between positional and keyword arguments and their correct ordering is essential for writing syntactically valid and maintainable Python code.

Key takeaways include the importance of adhering to the argument order when calling functions: positional arguments should always precede keyword arguments. This rule ensures clarity and consistency in function calls, making the code easier to read and debug. Additionally, developers should be mindful when refactoring or modifying function calls to avoid inadvertently reversing this order, which can lead to syntax errors and hinder program execution.

In summary, mastering the proper use of positional and keyword arguments not only helps prevent syntax errors like the “positional argument follows keyword argument” but also contributes to writing clean, efficient, and error-free Python code. Awareness and careful attention to argument placement are fundamental skills for any Python programmer aiming for professional-quality code.

Author Profile

Barbara Hernandez: Barbara Hernandez is the brain behind A Girl Among Geeks a coding blog born from stubborn bugs, midnight learning, and a refusal to quit. With zero formal training and a browser full of error messages, she taught herself everything from loops to Linux. Her mission? Make tech less intimidating, one real answer at a time.

Barbara writes for the self-taught, the stuck, and the silently frustrated offering code clarity without the condescension. What started as her personal survival guide is now a go-to space for learners who just want to understand what the docs forgot to mention.