Why Does `.join()` Produce Unexpected Results When Concatenating a String and a Bytes-like Object?

What is .join() and How Does It Function in Concatenating Strings?

Python's .join() method is commonly employed for string concatenation. However, upon testing the example provided, it yielded unexpected results. Understanding the purpose and usage of this method can clarify why it behaved as it did.

The .join() method operates on a string and takes an iterable of elements as an argument. It inserts the specified string between each element of the iterable during concatenation.

For instance:

",".join(["a", "b", "c"])
# Output: 'a,b,c'

In the example provided:

array.array('c', random.sample(string.ascii_letters, 20 - len(strid)))
.tostring().join(strid)

The tostring() method converts the array of characters into a bytes-like object. This bytes-like object is then used as the input for .join() along with the string "595".

However, the key point to understand is that the string "595" is treated as a list of its individual characters, i.e. ["5", "9", "5"]. Therefore, ".join()" operates on this list, inserting the bytes-like object between each character:

5wlfgALGbXOahekxSs9wlfgALGbXOahekxSs5
^                 ^                 ^

Each "5", "9", "5" in the output indicates the insertion of the bytes-like object between the characters of the original string.

To achieve the intended concatenation, the operator can be used instead:

array.array('c', random.sample(string.ascii_letters, 20 - len(strid)))
.tostring() + strid

This approach directly concatenates the bytes-like object with the string "595", producing the desired output without any insertions between characters.

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