Why does `.join()` insert separators when concatenating strings?

Understanding the Python .join() Method

Python's .join() method is a powerful tool for string concatenation, often preferred over alternatives like and str.format.

Question: Why does .join() behave unexpectedly when concatenating strings?

Answer: The .join() method takes a list of strings and inserts the provided separator string between each element. For example:

",".join(["a", "b", "c"]) == "a,b,c"

In the given example:

strid = repr(595)
array.array('c', random.sample(string.ascii_letters, 20 - len(strid)))
.tostring().join(strid)

The string "595" is converted into a list of characters: ["5", "9", "5"]. Then, the separator string is inserted between each character, resulting in the output:

5wlfgALGbXOahekxSs9wlfgALGbXOahekxSs5

Solution: To concatenate strings without inserting a separator, use instead:

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

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