In Python, why isn't it a syntax error when a list of strings is defined without comma separators?

Question

When coding in Python, I found that defining a list of strings without separating the strings with a comma is not a syntax error. When running this code:

x = ['a', 'b', 'c'] # this is the proper one
y = ['a' 'b' 'c'] # commas missing!
print(x) # results in ['a', 'b', 'c']
print(y) # results in ['abc']

However, if I run this code, Python complains about it.

a = 'a'
b = 'b'
c = 'c'
z = [a b c] # SyntaxError: invalid syntax. Perhaps you forgot a comma?

Instead of reporting an error, Python simply assumes that the three non-comma-separated strings should be combined into the same string. What are the reasons for this? Is it common for people to write lists of strings without comma separators and intend for them to be the same string?

Answer 1

The reason the first example does not error is because Python will automatically concatenate adjacent string literals. This happens inside and outside of lists.

As mentioned in the Python documentation, this:

can be used to reduce the number of backslashes needed, to split long strings conveniently across long lines, or even to add comments to parts of strings[.]

This feature was once considered for removal, however, "[t]here wasn’t enough support in favor, the feature to be removed isn’t all that harmful, and there are some use cases that would become harder."

As to why implicit string concatenation exists:

Many Python parsing rules are intentionally compatible with C. This is a useful default, but Special Cases need to be justified based on their utility in Python. ...

In C, implicit concatenation is the only way to join strings without using a (run-time) function call to store into a variable.

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