How can I test Perl applications using the system time?
I have a web application where I want to run some system tests and for that I will need to move the system time. The app uses DateTime all over.
Does anyone have any recommendations on how to change the time that DateTime-> now reports? The only thing that comes to mind is subclassing DateTime and messing around with all the lines of use, but that seems pretty invasive.
Note on answers:
All three will work fine, but Hook :: LexWrap is one - the one I chose because (a) I want to move the clock, not move it slightly (which is more the purpose of what Time :: Layout and friends do); (b) I use DateTime consistently and I am glad that errors appear if I did not accidentally use it; and (c) The :: LexWrap hook is simply more nifty than the symbol table hack for anything that does the same thing. (Also, it turns out to be dependent on some module that I already installed, so I didn't even have to use CPAN ...)
source to share
You can use Hook :: LexWrap to intercept the now () method.
use Hook::LexWrap;
use DateTime;
# Use real now
test();
{
my $wrapper = wrap 'DateTime::now',
post => sub {
$_[-1] = DateTime->from_epoch( epoch => 0 );
};
# Use fake now
test();
}
# use real now again
test();
sub test {
my $now = DateTime->now;
print "The time is $now\n";
}
source to share
Instead of taking a high-level approach and wrapping specifically DateTime
, you might want to look into the Test :: MockTime and Time :: Mock modules , which override low-level functions, which DateTime
, etc. use, and (with some luck) will do the right thing on any time sensitive code. This seems like a more reliable way of testing to me.
source to share
I think Hook :: LexWrap is too strong for this situation. It's easier to just override such a simple function.
use DateTime;
my $offset;
BEGIN {
$offset = 24 * 60 * 60; # Pretend it tomorrow
no warnings 'redefine';
sub DateTime::now
{
shift->from_epoch( epoch => ($offset + scalar time), @_ )
}
} # end BEGIN
You can replace my $offset
with our $offset
if you need to access $offset
from outside the file that contains this code.
You can tweak $offset
at any time if you want to change the idea of the DateTime of the current time during the run.
The calculation $offset
is likely to be more complex than shown above. For example, to set the "current time" to an absolute time:
my $want = DateTime->new(
year => 2009,
month => 9,
day => 14,
hour => 12,
minute => 0,
second => 0,
time_zone => 'America/Chicago',
);
my $current = DateTime->from_epoch(epoch => scalar time);
$offset = $want->subtract_datetime_absolute($current)->in_units('seconds');
But you probably want to calculate a fixed number of seconds to add to the current time so that the time afterwards will speed up. The problem with using add( days => 1 );
in an overridden method now
is that things like DST changes will cause the time to jump at the wrong pseudo-time.
source to share
When developing a new class that can be checked, ideally, the ideal solution is to be able to introduce new date objects.
However, for existing code using DateTime->now
and DateTime->today
possibly a suitable solution, see below. I am including it here as a way to do this without introducing Hook :: LexWrap as a dependency and without affecting the behavior globally.
{
no strict 'refs';
no warnings 'redefine';
local *{'DateTime::today'} = sub {
return DateTime->new(
year => 2012,
month => 5,
day => 31
);
};
say DateTime->today->ymd(); # 2012-05-31
};
say DateTime->today->ymd(); # today
source to share