How to create a "singleton" tuple with only one element

Question

In the below example I would expect all the elements to be tuples, why is a tuple converted to a string when it only contains a single string?

>>> a = [('a'), ('b'), ('c', 'd')]
>>> a
['a', 'b', ('c', 'd')]
>>> 
>>> for elem in a:
...     print type(elem)
... 
<type 'str'>
<type 'str'>
<type 'tuple'>

Merci

('a') just evaluates to 'a' – Parmesan

Wow - 3 correct answers in 3 minutes :) However, note the secret of ,: a = 1, 2, 3; print a – Dote

Brackets don't make a tuple, commas do. – Symon

@cdarke, except for the empty tuple (), which only consists in a pair of parentheses. – Innervate

True, or rather, False - just about all an empty tuple is good for (if you see what I mean). – Symon

Just to clarify the existing answers, parentheses are used for grouping, so ('a') is taken as an expression that evaluates to 'a'. Tuples need a comma to be a tuple, except the empty tuple, which is empty parentheses, (). – Bayard

tuple is an object delimited by comma not an object enclosed in parentheses. So a singleton tuple would be like ('a',) or tuple('a') – Hughhughes

Should also be noted that (True) and (False) are boolean types so that evaluating (1 == 2) is equivalent to 1 == 2 – Illeetvilaine

Answer 1

why is a tuple converted to a string when it only contains a single string?

a = [('a'), ('b'), ('c', 'd')]

Because those first two elements aren't tuples; they're just strings. The parenthesis don't automatically make them tuples. You have to add a comma after the string to indicate to python that it should be a tuple.

>>> type( ('a') )
<type 'str'>

>>> type( ('a',) )
<type 'tuple'>

To fix your example code, add commas here:

>>> a = [('a',), ('b',), ('c', 'd')]

             ^       ^

From the Python Docs:

A special problem is the construction of tuples containing 0 or 1 items: the syntax has some extra quirks to accommodate these. Empty tuples are constructed by an empty pair of parentheses; a tuple with one item is constructed by following a value with a comma (it is not sufficient to enclose a single value in parentheses). Ugly, but effective.

If you truly hate the trailing comma syntax, a workaround is to pass a list to the tuple() function:

x = tuple(['a'])

Congreve

This isn't very elegant though and looks kind of confusing. Is there any other way? – Handcrafted

That doesn't seem to work. For example: tuple("abc") (with or without extra comma) gives ('a', 'b', 'c'), while ("abc",) gives ('abc'). So tuple() seems not to be a viable option here. – Ptosis

@Ben Have a look at the docs. tuple accepts an iterable, which a string is (iterates over characters). If you insist on not using the trailing comma, then make an intermediate list: tuple(['abc']). – Congreve

Sure, I am just saying that the tuple function doesn't negate the need for the comma. – Ptosis

Anything more explicit variant of it in python 3.x by any chance? – Cornstalk

@matanster The language syntax has not changed in this regard. AFAIK the problem is the same for Python 2 and 3. – Congreve

Answer 2

Your first two examples are not tuples, they are strings. Single-item tuples require a trailing comma, as in:

>>> a = [('a',), ('b',), ('c', 'd')]
>>> a
[('a',), ('b',), ('c', 'd')]

Billiards

Answer 3

('a') is not a tuple, but just a string.

You need to add an extra comma at the end to make python take them as tuple: -

>>> a = [('a',), ('b',), ('c', 'd')]
>>> a
[('a',), ('b',), ('c', 'd')]
>>> 

Sidon

Answer 4

=> If you need to convert list to tuple which have one id as a element. then this solution will help you.

x_list = [1]

x_tuple = tuple(x_list)

=> You will get this

(1,)

=> so now append 0 into list and then convert it into tuple

=> x_list.append(0)

=> x_tuple = tuple(x_list)

(1, 0)

Honeybunch

...what does this have to do with the question being asked? – Naseby

Answer 5

Came across this page and I was surprised why no one mentioned one of the pretty common method for tuple with one element. May be this is version thing since this is a very old post. Anyway here it is:

>>> b = tuple(('a'))
>>> type(b)
<class 'tuple'>

Glauce

Strangely, this worked for me when the other answers didn't (passing a string value to a method, so i'm using a variable and not a raw string; maybe that's why (foo,) didn't work for me) – Dermato

This won't work as intended if the input string has several characters. Try, for example, tuple(('ab')) and the result will be ('a', 'b') instead of ('ab',). – Laveralavergne

Generally preferable way in that case is to supply a list, so as to avoid trailing comma syntax. e.g. tuple(['ab']) – Glauce

This method only works for strings of length 1, nothing else. The correct syntax is tuple([var]) – Embosser