[FIXED] Using buttons with @interact

Issue

EDIT: Re-written for clarity Sept. 8 2019 1330 UTC

In a Jupyter notebook, I want to explore dates chosen by a date picker widget. The way of doing this I already knew was to use @interact and make the datepicker widget an argument to the interact function, like this:

def explore_dates(d1=widgets.DatePicker(value=pd.to_datetime('2018-07-10', format='%Y-%m-%d'), description='Date')):

But I need buttons "Next Holiday" and "Previous Holiday" to move from the selected date to the next holiday. I already have an array of holidays stored as datetime.datetime objects.

So I tried adding the button objects to the arguments to the @interact function, like this:

@interact

def explore_dates(d1=widgets.DatePicker(value=pd.to_datetime('2018-07-10', format='%Y-%m-%d'), description='Date'),
                 prev_button = widgets.Button(description="Prev Holiday"),
                 next_button = widgets.Button(description="Next Holiday")):

That didn't work. Error says the line with the button widget "cannot be transformed into a widget". Hmm, I thought it already was a widget...

My next thought was to put the button code inside the function, rather than as an argument:

def explore_dates(d1=widgets.DatePicker(value=pd.to_datetime('2018-07-10', format='%Y-%m-%d'), description='Date')):

prev_button = widgets.Button(description="Prev Holiday")
next_button = widgets.Button(description="Next Holiday")
prev_button.on_click(prev_holiday_callback)
next_button.on_click(next_holiday_callback)
box=widgets.HBox([prev_button,next_button])
display(box)

That got the buttons on the screen and I can push them and my callback routines run. But when the datepicker is set up (per above) as an argument to @interact, it appears that it is no longer possible to reset its date using its .value attribute. (See code above) trying to set d1.value results in an error.

I suppose I could get rid of @interact entirely, and just put the datepicker and both buttons into the mainline code. But I don't know how to reproduce the function of @interact at that point. Short of an infinite loop that waits for a click or observe event to come from one of the widgets, other than @interact, I don't know how to tell Python/Jupyter "hey, just chill out until a widget event causes a callback to wake up the code".

In case it's helpful, here are my callback routines for the buttons.

def button_click_common():
    global button_flag
    global button_date
    global holidays
    
    for i in range(len(holidays)):
        if(holidays[i]<button_date): # haven't passed the prior holiday yet
            continue
        
        # Now holidays[i] has to either be same as or after button_date
        
        prior_holiday = i-1 # prior holiday is the one we just passed that was prior
        next_holiday = i    # assume at first we are between dates
        if holidays[i]==button_date:
            next_holiday +=1        # if we were already on a holiday date, next becomes the following one
        return prior_holiday, next_holiday

def prev_holiday_callback(_):
    global button_flag
    global button_date
    global holidays
    
    prior_holiday,_ = button_click_common()
    
    button_date = holidays[prior_holiday]
    button_flag = True
    
def next_holiday_callback(_):
    global button_flag
    global button_date
    global holidays
    
    _,next_holiday = button_click_common()
    
    button_date = holidays[next_holiday]    
    button_flag = True  

The idea here is that the callback updates button_date to the date of the next holiday, and sets the flag button_flag. Then code (not shown) in the explore_dates function was going to test the flag. But I can't figure out how to update the datepicker widget with the new date when it is defined as an argument to the @interact function.

Feels like I'm going about this wrong. Any advice or guidance welcome...

Thanks in advance!

Solution

I'm not sure I understand the exact aim you're trying to achieve, but interact works best with functions that take numeric inputs, rather than discrete inputs or python objects like dates.

I put together some code that would illustrate how I would use the forward and back buttons to update a datepicker, it's probably not exactly what you are looking for, but maybe you can describe the changes from there?

import ipywidgets as ipyw
from datetime import date

idx = 0
date_picker = ipyw.DatePicker()
date_picker.value = month_starts[idx]
start_button = ipyw.Button(description='Prev')
end_button = ipyw.Button(description='Next')

display(date_picker, start_button, end_button)

def change_date(button):
    global idx
    if button.description=='Prev':
        if idx-1<0:
            pass
        else:
            date_picker.value = month_starts[idx-1]
            idx -= 1

    elif button.description=='Next':
        if idx+1>=len(month_starts):
            pass
        else:
            date_picker.value = month_starts[idx+1]
            idx += 1

start_button.on_click(change_date)
end_button.on_click(change_date)

Answered By - ac24