With Python, concatenating strings with += in a loop creates many intermediate strings, leading to high memory usage and performance degradation. Prefer using f-strings (formatted string literals) or str.join() for better performance and lower environmental impact.

Non Compliant Code Example

city = "New York"
street = "5th Avenue"
zip_code = "10001"
address = ""
address += city + ", " + street + ", " + zip_code  # Noncompliant: inefficient string concatenation

In this example, the += operation creates a new string object for each concatenation. When done repeatedly (e.g., in loops), this leads to excessive memory allocations and slower performance.

Compliant Solution

# Using f-string for readability and performance
city = "New York"
street = "5th Avenue"
zip_code = "10001"
address = f"{city}, {street}, {zip_code}"
# or using str.join() for multiple string concatenations
parts = ["New York", "5th Avenue", "10001"]
address = ", ".join(parts)

These approaches avoid the repeated creation of new string objects. str.join() builds the final string in a single operation, and f-strings are compiled into efficient bytecode.

Relevance Analysis

This rule applies to any Python application performing repeated or large-scale string concatenation (e.g., log generation, data serialization, HTML template generation).

Configuration

  • Processor: Intel® Core™ Ultra 5 135U, 2100 MHz, 12 cores, 14 logical processors

  • RAM: 16 GB

  • CO2 Emissions Measurement: Using CodeCarbon

Context

Two string-building techniques were benchmarked: - Non-compliant: += string concatenation in a loop - Compliant: str.join() or f-string formatting

Metrics assessed: - Execution time - Carbon emissions

Impact Analysis

concat

  • CO₂ Emissions: Reduced by over 10× when using str.join() instead of +=

  • Energy Efficiency: Significantly improved due to lower memory allocations and faster execution

These results demonstrate that even small code patterns, like string concatenation, can have a measurable impact when scaled in production environments.

Conclusion

Replacing += in string concatenation with f-strings or str.join(): - Reduces memory overhead - Improves runtime performance - Decreases CO₂ emissions

References