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How to Resolve Python Error "SyntaxError: cannot assign to function call here"

The SyntaxError: cannot assign to function call here is a common Python error indicating a fundamental misuse of the assignment operator (=). It means you've placed a function call (which executes code and potentially returns a value) on the left-hand side of an assignment, where Python expects a target variable or location to store a value. Often, the error message helpfully suggests: "Maybe you meant '==' instead of '='?".

This guide explains the causes of this syntax error and provides clear solutions.

Understanding Assignment (=) vs. Function Calls​

  • Assignment (=): The single equals sign assigns the value calculated on the right side to the variable or target specified on the left side. The left side must be something that can receive a value (like a variable name, x, my_list[0], my_dict['key']). 
    result = 5 * 2 # Assign the value 10 to the variable 'result'
  • Function Call (function_name()): Parentheses () after a name execute the function. A function call is an expression that typically evaluates to a return value (or None if the function doesn't explicitly return anything). It is not a storage location itself.

The error occurs because you cannot assign a value to the result produced by a function call.

Primary Cause: Assigning to a Function Call​

The Problem: function() = value​

This is the most direct cause of the error. You're trying to force the result of a function call to be equal to some other value using the assignment operator.

def calculate_value():
  return 42

# ⛔️ SyntaxError: cannot assign to function call here. Maybe you meant '==' instead of '='?
# Cannot assign the string 'new value' TO the result of calculate_value()
calculate_value() = 'new value'

The Solution: variable = function()​

To store the result returned by a function, assign it to a variable. The function call belongs on the right-hand side of the assignment.

def calculate_value():
  return 42

# ✅ Assign the function's return value to a variable
result_variable = calculate_value()

print(f"The result is: {result_variable}") # Output: The result is: 42

Confusion with Comparison (==)​

Sometimes, the error arises because you intended to compare the result of a function call with another value but mistakenly used the assignment operator (=) instead of the comparison operator (==).

The Problem: Using = to Compare Function Results​

def get_status():
  return "active"

# Attempting comparison with =
# ⛔️ SyntaxError: invalid syntax (often caught this way in 'if', but conceptually same error)
# Or sometimes "cannot assign to function call" if the structure is more complex.
if get_status() = "active":
    print("Status is active")

The Solution: Use == for Comparison​

Always use the double equals sign (==) to check if two values are equal.

def get_status():
  return "active"

# ✅ Correct comparison using ==
if get_status() == "active":
    print("Status check: Status is active.") # This runs
else:
    print("Status check: Status is not active.")

Mistaking Parentheses () for Indexing/Keys []​

Parentheses () are used for function calls (and grouping expressions, tuple creation), while square brackets [] are used for accessing elements by index (lists, tuples, strings) or by key (dictionaries). Using () incorrectly for assignment or access with lists/dicts can sometimes lead to this error if Python interprets variable(something) as a function call attempt.

Dictionary Access/Assignment​

my_dictionary = {}

# ⛔️ SyntaxError: cannot assign to function call here.
# my_dictionary('key') looks like calling my_dictionary as a function
my_dictionary('new_key') = 'some_value'

The Correct solution is to use square brackets [] for keys

my_dictionary = {}

my_dictionary['new_key'] = 'correct_value'
print(f"Dictionary after correction: {my_dictionary}")  # Output: {'new_key': 'correct_value'}
print(f"Accessing value: {my_dictionary['new_key']}")   # Output: correct_value

Output:

Dictionary after correction: {'new_key': 'correct_value'}
Accessing value: correct_value

List Index Access/Assignment​

my_list = [10, 20, 30]

# ⛔️ SyntaxError: cannot assign to function call here.
# my_list(0) looks like calling my_list as a function
my_list(0) = 5

The correct solution is to use square brackets [] for indices

my_list = [10, 20, 30]
my_list[0] = 5 # Assign to index 0
print(f"List after correction: {my_list}")  # Output: [5, 20, 30]
print(f"Accessing index 0: {my_list[0]}")   # Output: 5

Output:

List after correction: [5, 20, 30]
Accessing index 0: 5

Incorrect Loop/Comprehension Syntax​

This error can appear if the syntax within a list comprehension or generator expression incorrectly places a function call where an iterable or loop variable is expected.

def get_source_string():
    return "abc"

# ⛔️ SyntaxError: cannot assign to function call
# The structure is wrong: Should be 'for char in get_source_string()'
result = [char for get_source_string() in char]

The correct comprehension syntax is the following:

def get_source_string():
    return "abc"

result_correct = [char for char in get_source_string()]     # Iterate over the *result* of the function call
print(f"Correct comprehension result: {result_correct}")    # Output: ['a', 'b', 'c']

Output:

Correct comprehension result: ['a', 'b', 'c']
note

Ensure the for variable in iterable part of your loop or comprehension is structured correctly, with the iterable (which can be a function call result) appearing after in.

Valid Variable and Function Names (Context)​

While not the direct cause of this error, remember that function calls require valid function names, and assignment requires valid variable names on the left. Variable/function names must start with a letter or underscore and contain only letters, numbers, and underscores.

Conclusion​

The SyntaxError: cannot assign to function call here clearly indicates that you are trying to use a function call (function()) on the left side of an assignment operator (=). Remember:

  • Function calls produce values; they are not storage locations.
  • Assign the result of a function call to a variable: variable = function().
  • Use the comparison operator (==) when checking if a function's result equals a value: if function() == value:.
  • Use square brackets ([]) for dictionary key access/assignment (my_dict['key'] = ...) and list index access/assignment (my_list[index] = ...), not parentheses ().
  • Double-check the syntax of loops and comprehensions to ensure correct iteration structure.

By understanding the role of the assignment operator and the nature of function calls, you can easily correct this syntax error.

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