Problem with list of strings in python

Question

Why on Earth doesn't the interpreter raise SyntaxError everytime I do this:

my_abc = ['a',
          'b',
          'c'
          'd',]

I just wanted to add 'c' to the list of strings, and forgot to append the comma. I would expect this to cause some kind of error, as it's cleary incorrect.

Instead, what I got:

>>> my_abc
 ['a', 'b', 'cd']

And this is never what I want.

Why is it automatically concatenated? I can hardly count how many times I got bitten by this behavior. Is there anything I can do with it?

Just to clarify*: I don't actually mind auto-concatenation, my problem has to do ONLY with lists of strings, because they often do much more than just carry text, they're used to control flow, to pass field names and many other things.

Answer 1

It's by design. It allows, for example, writing long string literals in several lines without using +.

Answer 2

Because two string literals side-by-side, delimited by whitespace, are concatenated. Since the strings are within a list, they are 'side-by-side'.

See: http://docs.python.org/reference/lexical%5Fanalysis.html#string-literal-concatenation

Answer 3

Because often people want to do something like this:

line = ("Here's a very long line, with no line breaks,"
        " which should be displayed to the user (perhaps"
        " as an error message or question box).")

It's easier to write this without having to manually concatenate strings. C, C++, and (I believe) Java and C# also have this behavior.

Answer 4

Is called "Implicit String Concatenation" and a PEP that proposed its removal was rejected: http://www.python.org/dev/peps/pep-3126/

Answer 5

As others said, it's by design.

Why is it so ? Mostly for historical reasons : C also does it.

In some cases it's handy because it reduce syntaxic noise and avoid adding unwanted spaces (inline SQL queries, complexes regexpes, etc).

What you can do about it ? Not much, but if it really happens often for you, try one of the following tricks.

• indent your list with coma at the beginning of the line. It's weird, but if you do so the missing comas become obvious.
• assign strings to variables and use variables list whenever you can (and it's often a good idea for other reasons, like avoiding duplicate strings).

• split your list: for list of words you can put the whole list inside only one string and split it like below. For more than 5 elements it's also shorter.

  'a b c d e'.split(' ').