String Concatenation in Python: Is There a Faster Way to Append Strings?

Concatenating Strings in Python: An Efficient Approach

Question: How can I efficiently append one string to another in Python? Is there a faster alternative to the following code?

<code class="python">var1 = "foo"
var2 = "bar"
var3 = var1 + var2</code>

Answer:

CPython, the main Python implementation, now optimizes string concatenation by attempting to extend the string in place when only one reference to a string is present. This optimization results in amortized O(n) time complexity.

For instance, the following code:

<code class="python">s = ""
for i in range(n):
    s += str(i)</code>

which used to have a time complexity of O(n^2), is now O(n).

Technical Details:

In the CPython implementation, the _PyBytes_Resize function is responsible for this optimization. It allows the resizing of strings without creating a new object, provided that only one module references the original string.

Performance Analysis:

Empirical testing demonstrates the significant performance improvement for string concatenation operations:

String Size	Concatenation Time (CPython)
10	1.85 usec
100	16.8 usec
1,000	158 usec
10,000	1.71 msec
100,000	14.6 msec
1,000,000	173 msec

Important Note:

This optimization is specific to CPython and may not be present in other Python implementations, such as PyPy or Jython. In these cases, string concatenation performance may differ from the CPython implementation.

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