why does python string formatting need a tuple? Why doesn't a list work?

Question

I was surprised that python's percent-style formatting won't accept a list and seems to only accept tuples. What is special about tuples here? Why does the list throw the error?

In [1]: '%s %s' % ('hello', 'kilojoules')                                                     
Out[1]: 'hello kilojoules'

In [2]: '%s %s' % ['hello', 'kilojoules']                                                     
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-2-764f27542c69> in <module>
----> 1 '%s %s' % ['hello', 'kilojoules']

TypeError: not enough arguments for format string

Trivial answer: because that's how it's specified. Why this decision was made is off-topic for Stack Overflow; maybe try comp.lang.python? — Amadan
– Amadan, Commented Jun 24, 2019 at 1:07

Shadow · Accepted Answer · 2019-06-24 01:12:45Z

3

The way you are using to format strings is a special syntax. In the list example you show, it is converting the non-tuple (a list) into a string before attempting to use it for formatting purposes.

You can do what you're trying to do by using the format method.

In [1]: "{} {}".format(*['hello', 'kilojoules'])

Doing it this way is recommended anyway, as it is preferred.

edited Jun 24, 2019 at 1:12

answered Jun 24, 2019 at 1:00

Shadow

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1 Comment

juanpa.arrivillaga Over a year ago

It may not be deprecated, but it is specifically discouraged in the docs. Unless you are maintaining an old code base, you should stick to either str.format or f-string literals

Nakor · Accepted Answer · 2019-06-24 01:08:46Z

2

Also, with the latest updates in python3.6, you can use f-Strings like this:

> great = "hello"
> name = "kilojoules"

> f"{great} {name}"
'hello kilojoules'

This website gives a good summary of the different ways to do it.

answered Jun 24, 2019 at 1:08

Nakor

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Comments

Grismar · Accepted Answer · 2019-06-24 01:41:01Z

1

The documentation (https://python-reference.readthedocs.io/en/latest/docs/str/formatting.html) answers your question directly: "If format specifier requires a single argument, values may be a single non-tuple object. Otherwise, values must be a tuple with exactly the number of items specified by the format string, or a single mapping object (for example, a dictionary)." A list is neither, so it won't do.

Like others have pointed out, you could just use .format for a more modern approach, or even the still more modern f-string, which is as concise as your original example:

one_way = '{} {}'.format(*['hello', 'kilojoules'])
lst = ['hello', 'kilojoules']
another_way = f'{lst[0]} {lst[1]}'

edited Jun 24, 2019 at 1:41

answered Jun 24, 2019 at 1:12

Grismar

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2 Comments

Shadow Over a year ago

Your example of the 'more modern f string' is missing the f bit :P

Grismar Over a year ago

Thanks @shadow - nasty oversight :); I blame the painful way of moving code between my IDE and SO (although I'm really the only one to blame); fixed it.

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