string literals in a list. Not what I expected.

[tycarac]

Hi

I saw unexpected Python behavior (to me) with string literals in a list. If I miss a comma in the list sequence, then the two string literals are merged into on string. What I had expected was a syntax error.

In the example below, if I remove the comma between the 'e' and 'i', the list merges the two into 'ei'. The missing comma acts as an implicit + operator.

If I miss a comma between a variable a string literal, or put two commas in a row, I get the Python syntax error I expect.

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C:\Users\me>python Python 3.8.1 (tags/v3.8.1:1b293b6, Dec 18 2019, 23:11:46) [MSC v.1916 64 bit (AMD64)] on win32 Type "help", "copyright", "credits" or "license" for more information.   >>> ['a', 'e', 'i', 'o', 'u'] ['a', 'e', 'i', 'o', 'u']   >>> ['a', 'e' 'i', 'o', 'u'] ['a', 'ei', 'o', 'u']   >>> ['a', 'e' + 'i', 'o', 'u'] ['a', 'ei', 'o', 'u']   >>> s = 'b'; ['a', 'e', s, 'i', 'o', 'u'] ['a', 'e', 'b', 'i', 'o', 'u']   >>> s = 'b'; ['a', 'e', s 'i', 'o', 'u']   File "<stdin>", line 1     s = 'b'; ['a', 'e', s 'i', 'o', 'u']                           ^ SyntaxError: invalid syntax   >>> s = 'b'; ['a', 'e', , 'i', 'o', 'u']   File "<stdin>", line 1     s = 'b'; ['a', 'e', , 'i', 'o', 'u']                         ^ SyntaxError: invalid syntax   >>>

I want to confirm this merging of literals is correct Python behavior.

To me, Python interpreting the missing comma as an implicit append operator seems error prone. It would be hard to spot a missing comma would be hard to spot in a large list of string literals and a cause of programming bugs.

[perfringo]

Python documentation: String literal concatenation

[Larz60+]

This is more than I want to read right now, but perhaps you will find your answer:
https://docs.python.org/3/library/stdtyp...operations

I was unaware of this behaviour, thus don't know if legal or not. I would expect that after all this time, having not been changed, that there is a perfectly good reason for this behaviour.
If you don't find one, place a query on python.org to the authors of python.

[tycarac]

@perfringo
Thanks for the link. So yes it is legal.

Though in my opinion it is dangerous and easily introduces errors. A pity the '+' operator is only required for runtime but not for compile time. A gotcha for Python coding.