# Mysterious Invalid Syntax in Field Calculation

I'm trying to calculate a field in Model Builder using the Calculate Field tool. I'm using the following code block and expression in the calculation, but am getting an "invalid syntax (, line 1)" error.

# Code Block:
def get_block(block_id):
block_only = str(block_id)[1:]
if block_only[-1].isalpha():
return block_only.zfill(3)
else:
return block_only.zfill(2)

# Expression:
get_block(%block_id%)


%block_id% is an in-line substitution that points to a string variable I create as the output of a custom Python script tool. Any ideas what the error is?

You need to quote the in-line substitution to pass the string variable.

 get_block('%block_id%')

 get_block("%block_id%")