I keep seeing the same question about the new default argument for the templating. This post will cover all possible ways to define default.
All the methods that were changed:
acos
as_timestamp
asin
atan
atan2
cos
float
log
round
sin
sqrt
strptime
tan
timestamp_custom
timestamp_local
timestamp_utc
Each of them have the exact same default functionality but a different number of arguments. There are multiple ways to use these functions, and this is where all the confusion lies.
Understanding arguments on filters and functions.
Letās take for example a fake function called my_fake_function. The documentation spells out the function as:
my_fake_function as a function
my_fake_function(value, mode="a", flag=False)
\ / \ / \ /
\ / \ / \ /
 \ / \ /
required arugment  \ /
 
optional keyword argument optional keyword argument
my_fake_function as a filter
value  my_fake_function(mode="a", flag=False)
\ / \ / \ /
\ / \ / \ /
 \ / \ /
required arugment  \ /
 
optional keyword argument optional keyword argument
From this documentation line, I know the following information:

my_fake_function
has 3 arguments:value
,mode
, andflag
. 
value
is a required argument. You can tell because it does not have a value set on it.mode
andflag
are optional arguments. This means you donāt need to provide the argument tomy_fake_function
in order for it to work. 
mode
is an optional argument because it has a value attached to it already. You can also call outmode
by name when passing in the information. This is because it has a default value which makes it a keyword argument. If you donāt providemode
,my_fake_function
will use'a'
asmode
. 
flag
is an optional argument because it has a value attached to it already. You can also call outflag
by name when passing in the information. This is because it has a default value which makes it a keyword argument. If you donāt provideflag
,my_fake_function
will useFalse
forflag
.
Example 1
In this example we will use value of 4
, mode
of "a"
and flag
of True
. Because mode is omitted, my_fake_function
will use "a"
because the documentation said so.
{{ my_fake_function(4, flag=True) }}
This is also valid, instead of naming flag=True
, you can simply provide all 3 arguments.
{{ my_fake_function(4, "a", True) }}
You can also name all optional arguments.
{{ my_fake_function(4, mode="a", flag=True) }}
Example 2
In this example will use value
of 7
, mode
of "b"
and flag
of False
. Keep in mind that we can omit flag
because itās False
, but you can also provide it if you want. This gives us 4 options on how to write this out.
{{ my_fake_function(7, "b") }}
{{ my_fake_function(7, "b", False) }}
{{ my_fake_function(7, mode="b") }}
{{ my_fake_function(7, mode="b", flag=False) }}
Example 3
This will use value
of 9
, mode
of ācā and flag
of True
. Because we are changing all 3 arguments, we have to provide all 3. Leaving us with the only 2 valid ways of writing it.
{{ my_fake_function(7, "c", True) }}
{{ my_fake_function(7, mode="c", flag=True) }}
Notes
 You cannot use the argument name when specifying a required argument. In this example, that means you cannot use
value=4
when specifying the required arugmentvalue
.  Not all functions are filters. The documentation will tell you what is a filter and what is a function.
 Not all filters are functions. The documentation will tell you what is a filter and what is a function.
Using them as a function.
2 arguments
breakdown
{{ function(value_for_argument1, value_for_argument2) }}
examples
# returns acos of 1, if fails returns 0
{{ acos(1, 0) }}
# returns as_timestamp of 1, if fails returns 0
{{ as_timestamp(1, 0) }}
# returns asin of 1, if fails returns 0
{{ asin(1, 0) }}
# returns atan of 1, if fails returns 0
{{ atan(1, 0) }}
# returns cos of 1, if fails returns 0
{{ cos(1, 0) }}
# returns 1.0, if fails returns 0
{{ float('1', 0) }}
# returns sin of 1, if fails returns 0
{{ sin(1, 0) }}
# returns sqrt of 1, if fails returns 0
{{ sqrt(1, 0) }}
# returns tan of 1, if fails returns 0
{{ tan(1, 0) }}
3 arguments
breakdown
{{ function(value_for_argument1, value_for_argument2, value_for_argument3) }}
examples
# returns time @ 10 am, if fails returns 0
{{ strptime("10:00", "%H:%M", 0) }}
# returns log of 1 with base 10, if fails returns 0
{{ log(1, 10, 0) }}
# returns atan2 of 1 at 2, if fails returns 0
{{ atan2(1, 2, 0) }}
# returns rounds 1.23 down precision of 0, if fails returns 0
{{ round(1.43, "floor", 0) }}
3 arguments but omitting the 2nd argument
breakdown
{{ function(value_for_argument1, argument3 = value_for_argument3) }}
examples
# returns log of 1 with base e because base (2nd argument) defaults to e, if fails returns 0
{{ log(1, default = 0) }}
# returns rounds 1.43 to even with a precision of 0
# because function (2nd argument) defaults to roundtoeven, if fails returns 0
{{ round(1.43, default = 0) }}
Using them as filters
2 arguments
breakdown
{{ value_for_argument1  function(value_for_argument2) }}
examples
# returns as_timestamp of 1, if fails returns 0
{{ 1  as_timestamp(0) }}
# returns 1.0, if fails returns 0
{{ '1'  float(0) }}
# returns fully formatted time string 1 second past
# midnight January 1st 1970, if fails returns 0
{{ 1  timestamp_local(0) }}
# returns fully formatted time string 1 second past
# midnight January 1st 1970, if fails returns 0
{{ 1  timestamp_utc(0) }}
3 arguments
breakdown
{{ value_for_argument1  function(value_for_argument2, value_for_argument3) }}
examples
# returns time @ 10 am, if fails returns 0
{{ "10:00"  strptime("%H:%M", 0) }}
# returns rounds 1.23 down precision of 0, if fails returns 0
{{ 1.43  round("floor", 0) }}
3 arguments but omitting the 2nd argument
breakdown
{{ value_for_argument1  function(argument3 = value_for_argument3) }}
examples
# returns rounds 1.23 down precision of 0, if fails returns 0
{{ 1.43  round(default=0) }}
4 arguments
breakdown
{{ value_for_argument1  function(value_for_argument2, value_for_argument3, value_for_argument4) }}
examples
# returns "00:00:01", which is 1 second past
# midnight January 1st 1970 in local time, if fails returns 0
{{ 1  timestamp_custom("%H:%M:%S", True, 0) }}
4 arguments omitting the 3rd argument
breakdown
{{ value_for_argument1  function(value_for_argument2, value_for_argument3, value_for_argument4) }}
examples
# returns "00:00:01", which is 1 second past
# midnight January 1st 1970 in local time (local_time defaults to True), if fails returns 0
{{ 1  timestamp_custom("%H:%M:%S", default = 0) }}
Hopefully, you can see there are many ways to define default for all the functionand filters. Everything covered in this post is valid. Keep in mind that some functions are not filters and some filters are not functions. All of this is covered in the documentation. This post does not mix and match filters. Everything in the filter section is a filter, anything that is missing is not a filter. Same goes for the function section.