[FIXED] <lambda>() takes 1 positional argument but 2 were given

In this article, we have explained the reason behind the Python error "<lambda>() takes 1 positional argument but 2 were given" and have presented multiple ways in which this error can be fixed.

Table of contents:

1. Understanding the error
2. Fix 1: Basic case
3. Fix 2: Error with reduce()
4. Concluding Note

Understanding the error

While running a Python code which uses lambda you may encounter the following error:

Traceback (most recent call last):
  File "opengenus.py", line 4, in <module>
    a = reduce(f, list)
TypeError: <lambda>() takes 1 positional argument but 2 were given

The problem is that the lambda defined in the code takes 1 input parameter but during the invocation of the lambda function 2 input parameters are passed and hence, the error.

The fix will involve:

• Updating the lambda to take 2 inputs
• OR, update the way lambda is called so correct number of input arguments are passed.

There could be other variants of this error when there is a mismatch between the number of parameters expected by the lambda and the number of parameters passed to the lambda. For example:

TypeError: <lambda>() takes 1 positional argument but 2 were given
TypeError: <lambda>() takes 1 positional argument but 2 were given

We will explore how to fix this error.

Fix 1: Basic case

Consider the following Python code which defined a lambda that takes 1 input and adds one to it but while calling the lambda, we are passing two input values. This is the case where the error will come.

f = lambda x: x+1
a = f(4, 3)
print(a)

Run it as:

python code.py

Error:

Traceback (most recent call last):
  File "opengenus.py", line 4, in <module>
    a = reduce(f, list)
TypeError: <lambda>() takes 1 positional argument but 2 were given

The fix is simple. Call the lambda with one element at a time as the lambda is designed to take 1 input parameter. In some code, lambda can take 2 or more input parameters as well.

The fixed Python code is as follows:

f = lambda x: x+1
a = f(4)
print(a)

Output:

5

Similarly, make sure that the lambda is not called with less number of input parameters than expected by the lambda.

Fix 2: Error with reduce()

One case when the error may come is during the use of reduce(). Reduce() built-in function is frequently used to call a lambda function on repeatedly on each element of an iterable.

Consider the following code:

from functools import reduce
list = [91, 12, 63, 5] 
f = lambda x: print(x)
a = reduce(f, list)

Run it as:

python code.py

Error:

Traceback (most recent call last):
  File "opengenus.py", line 4, in <module>
    a = reduce(f, list)
TypeError: <lambda>() takes 1 positional argument but 2 were given

This issue is reduce() invokes the lambda with two parameters and hence, a fix will be to update the lambda to take 2 input parameters. Consider the following Python code which fixes the issue:

from functools import reduce
list = [91, 12, 63, 5] 
f = lambda x, y: print(x, y)
a = reduce(f, list)

Output:

91 12
None 63
None 5

Concluding Note

With this article at OpenGenus, you must have the complete idea of how to fix this error of <lambda>() takes 1 positional argument but 2 were given.

Always check if the definition of a function and if you are calling the function as expected.