Why does adding a trailing comma after an expression create a tuple?

Why does adding a trailing comma after an expression create a tuple with the expression's value? E.g. in this code:

>>开发者_StackOverflow中文版> abc = 'mystring',
>>> print(abc)
('mystring',)

Why is the printed output ('mystring',), and not just mystring?

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It is the commas, not the parentheses, which are significant. The Python tutorial says:

A tuple consists of a number of values separated by commas

Parentheses are used for disambiguation in other places where commas are used, for example, enabling you to nest or enter a tuple as part of an argument list.

See the Python Tutorial section on Tuples and Sequences

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Because this is the only way to write a tuple literal with one element. For list literals, the necessary brackets make the syntax unique, but because parantheses can also denote grouping, enclosing an expression in parentheses doesn't turn it into a tuple: you need a different syntactic element, in this case the comma.

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Make sure to read this great answer by Ben James.

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Tuples are not indicated by the parentheses. Any expression can be enclosed in parentheses, this is nothing special to tuples. It just happens that it is almost always necessary to use parentheses because it would otherwise be ambiguous, which is why the __str__ and __repr__ methods on a tuple will show them.

For instance:

abc = ('my', 'string')
abc = 'my', 'string'

What about single element tuples?

abc = ('mystring',)
abc = 'mystring',

So in effect what you were doing was to create a single element tuple as opposed to a string.

The documentation clearly says:

An expression list containing at least one comma yields a tuple. The length of the tuple is the number of expressions in the list. The expressions are evaluated from left to right.

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Unpacking multi-element tuple:

a, b = (12, 14)

print(type(a))

Output:

int

Unpacking single-element tuple:

a, = (12, )

print(type(a))

Output:

int

Otherwise:

a = (12,)

print(type(a))

Output:

tuple

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In the question's example, you assigned the variable 'abc' to a Tuple with a length of 1.

You can do multiple assignments with this similar syntax:

x,y = 20,50

Also note that the print statement has a special understanding for ending a print statement with a comma; This tells print to omit the trailing newline.

print 'hello',
print 'world'

result:

hello world

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I was somewhat confused about the application of the comma, as you also apply a comma to make a list instead of tuple, but with a different variable assignment.

Hereby, a simple example that I made of how to create a tuple or a list.

abc = 1,2,3 # prints a tuple: (1, 2, 3)
*abc, = 1,2,3 # prints a list: [1, 2, 3]