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I have a list mylist = [1, 2, 3] and a number n which I want to print in a specific way:

{n} {book or books} read ok: {mylist elements one by one separated by commas)

for example:

1 book read ok: 1 , 2, 3

2 books read ok: 1 , 2, 3

The decision of printing book or books depends on n, i.e. if n is 1 then print book, else print books. The code for achieving that is:

print("{:s}", "book" if n == 1 else "books")

For printing the elements of the list the code is:

print(*mylist, sep=' , ')

But, I am having problems trying to combine all of this into the desired output with including .format for displaying n as described above.

Here is what I have tried so far:

print("{0} {:s} read ok: {1}".format(n, *mylist, sep=' , '), "book" if n == 1 else "books")

which gives ValueError: cannot switch from manual field specification to automatic field numbering

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  • 1
    And what don't you understand about that message? You have three placeholders, {0}, {:s} and {1}, and Python can't figure out what you expected to go where! Commented Jun 16, 2016 at 11:39
  • @jonrsharpe Well, however on-point and informative the error message was, I did not know how to overcome it and achieve my goal. Hence, my question. Commented Jun 16, 2016 at 12:12

1 Answer 1

Reset to default
2

Dissected call to print():

print(
    "{0} {:s} read ok: {1}".format(
        n,  # 1st argument to format
        *mylist,  # mylist unpacked as positional arguments to format
        sep=' , '  # a keyword argument to format
    ),  # Formatted string, the 1st argument to print
    "book" if n == 1 else "books"  # 2nd argument to print
)

The error is a result of mixing manual and automatic field specifications, as it very clearly states. Don't mix manual ("{0}" etc). with automatic ("{}"). In your string you have 0, automatic and 1. There's no sane way to decide which positional argument the automatic should be, and so the error is raised.

From the documentation:

If the numerical arg_names in a format string are 0, 1, 2, ... in sequence, they can all be omitted (not just some)

A working print in this case could be:

print("{} {} read ok: {}".format(
    n,
    "book" if n == 1 else "books",
    ', '.join(str(i) for i in mylist)
))

The format string is equal to:

"{0} {1} read ok: {2}"
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2 Comments

Quick follow-up: could you briefly explain why the {:s} is not needed? Confused how it did work previously in isolation with that :s
Depends on your isolated test. The part after the colon is the format specification, e.g. in "{:s}" it is the s. s is the default presentation type for string types and may be omitted. Hence it equals "{}" for string types. For integers the default presentation type is d, for example.

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