How are booleans formatted in Strings in Python?


I see I can’t do:

"%b %b" % (True, False)

in Python. I guessed %b for b(oolean). Is there something like this?

Asked By: Juanjo Conti



If you want True False use:

"%s %s" % (True, False)

because str(True) is 'True' and str(False) is 'False'.

or if you want 1 0 use:

"%i %i" % (True, False)

because int(True) is 1 and int(False) is 0.

Answered By: Desintegr
>>> print "%r, %r" % (True, False)
True, False

This is not specific to boolean values – %r calls the __repr__ method on the argument. %s (for str) should also work.

Answered By: danben

You may also use the Formatter class of string

print "{0} {1}".format(True, False);
print "{0:} {1:}".format(True, False);
print "{0:d} {1:d}".format(True, False);
print "{0:f} {1:f}".format(True, False);
print "{0:e} {1:e}".format(True, False);

These are the results

True False
True False
1 0
1.000000 0.000000
1.000000e+00 0.000000e+00

Some of the %-format type specifiers (%r, %i) are not available. For details see the Format Specification Mini-Language

Answered By: Wolf

To update this for Python-3 you can do this

"{} {}".format(True, False)

However if you want to actually format the string (e.g. add white space), you encounter Python casting the boolean into the underlying C value (i.e. an int), e.g.

>>> "{:<8} {}".format(True, False)
'1        False'

To get around this you can cast True as a string, e.g.

>>> "{:<8} {}".format(str(True), False)
'True     False'

Referring to Python string interpolation

In Python 3.6, string interpolation has been added with a new string literal f prefix

shouldBeTrue = True
Answered By: ChinLoong

To expand on the answer by phd, you can do all that without str()-ing your booleans in the format() call. There are some weird tricks you can exploit to do formatting for booleans with the new style of string interpolation. It also works for f-strings.

The bland, trivial case

'{},{}'.format(True, False)  # 'True,False'
f'{True},{False}'            # 'True,False'

Adding some formatting converts booleans to integers

Any padding length will do this. I am not sure why someone would want it to work this way.

'{:0},{:0}'.format(True, False)  # '1,0'
f'{True:0},{False:0}'            # '1,0'

'{:>5},{:>5}'.format(True, False)  # '    1,    0'
f'{True:>5},{False:>5}'            # '    1,    0'

String coercion can bring back the normal text

Note the !s in there. This is the most direct equivalent to %s. There’s also !r for __repr__() and !a for ASCII, but they’re not particularly interesting for booleans.

'{!s:>5},{!s:>5}'.format(True, False)  # ' True,False'
f'{True!s:>5},{False!s:>5}'            # ' True,False'
Answered By: Michael