The error message:
ValueError: zero-length field name in format
occurs when using Python’s .format() method incorrectly, especially in Python 2.7 or early versions of Python 3.
1. Cause of the Error
Cause 1: Using Empty {} in Older Python Versions
In Python 2.7 and early Python 3 versions (before 3.1), you had to explicitly specify field numbers inside {} when using .format().
Incorrect Code (in older Python versions):
print("Hello, {} {}".format("John", "Doe")) # Error in Python < 3.1
Solution: Specify field numbers explicitly.
print("Hello, {0} {1}".format("John", "Doe")) # Works in all versions
Or upgrade to Python 3.1+ where empty {} is allowed.
print("Hello, {} {}".format("John", "Doe")) # Works in Python 3.1+
Cause 2: Incorrect Use of str.format() with Number Formatting
This error can also occur if you try to format numbers incorrectly.
Incorrect Code:
print("{:}".format(10)) # Error in older Python versions
Solution: Use an explicit field index.
print("{0:}".format(10)) # Works in all versions
Cause 3: Using format_map() with a Dictionary Without Keys
If you use .format_map() but don’t provide field names inside {}, it results in an error.
Incorrect Code:
data = {'name': 'Alice', 'age': 25}
print("Hello, {} is {} years old".format_map(data)) # Error: No keys
Solution: Use dictionary keys inside {}.
print("Hello, {name} is {age} years old".format_map(data))