How to perfectly convert one-element list to tuple

问题:

So I am trying to do this:

tuple([1])

The output I expect is :

(1)

However, I got this:

(1,)

But if I do this:

tuple([1,2])

It works perfectly! like this:

(1,2)

This is so weird that I don't know why the tuple function cause this result.

Please help me to fix it.

回答1:

This is such a common question that the Python Wiki has a page dedicated to it:

One Element Tuples

One-element tuples look like:

1,

The essential element here is the trailing comma. As for any expression, parentheses are optional, so you may also write one-element tuples like

(1,)

but it is the comma, not the parentheses, that define the tuple.

回答2:

That is how tuples are formed in python. Using just (1) evaluates to 1, just as much as using (((((((1))))))) evaluates to ((((((1)))))) to (((((1))))) to... 1.

Using (1,) explicitly tells python you want a tuple of one element

回答3:

What you are getting is a tuple. When there is only a single element, then it has to be represented with a comma, to show it is a tuple.

Eg)

>>> a = (1)
>>> type(a)
<type 'int'>
>>> a = (1,)
>>> type(a)
<type 'tuple'>
>>>

The reason is, when you do not use a comma when there is only one element, the interpreter evaluates it like an expression grouped by paranthesis, thus assigning a with a value of the type returned by the expression

回答4:

From the docs

6.2.3. Parenthesized forms

A parenthesized form is an optional expression list enclosed in parentheses:

parenth_form ::=  "(" [expression_list] ")" 

A parenthesized expression list yields whatever that expression list yields: if the list contains at least one comma, it yields a tuple; otherwise, it yields the single expression that makes up the expression list.

An empty pair of parentheses yields an empty tuple object. Since tuples are immutable, the rules for literals apply (i.e., two occurrences of the empty tuple may or may not yield the same object).

Note that tuples are not formed by the parentheses, but rather by use of the comma operator. The exception is the empty tuple, for which parentheses are required — allowing unparenthesized “nothing” in expressions would cause ambiguities and allow common typos to pass uncaught.

So (1,) really is a tuple

回答5:

(1) is just 1 in grouping parentheses - it's an integer. (1,) is the 1-element tuple you want.

回答6:

That is normal behavior in Python. You get a Tuple with one element. The notation (1,) is just a reminder that you got such a tuple.

回答7:

The output (1,) is fine. The , is to mark a single element tuple.

If

a = (1) 

a is really a integer

If

a = (1, )

Then a is a tuple.