Missing calendar module in python3.9 venv

Question

I am trying to run a python application inside python3.9 virtualenv. But I am getting the following error:

AttributeError: module 'calendar' has no attribute 'monthlen'

This is how I setup the environment:

sudo apt install python3.9-venv
python3.9 -m venv .venv/
source  .venv/bin/activate

I could see that 'calendar' module is not loaded from the virtual env. Instead it's loaded from the system python installation:

>>> import calendar
>>> print(calendar.__file__)
/usr/lib/python3.9/calendar.py
>>> 

Looks like "calendar" module is supposed to be part of the python installation, but is not there in the virtual env (cant locate any files in the virtual env with name containing "calendar" ) and is not available via pip. How can I get the calendar module and any other standard inbuilt modules I might need later?

Answer 1

calendar.monthlen was an undocumented Python 3.7 function, that has been renamed to calendar._monthlen in Python 3.8 onwards to inform you that it's a "private" function and you should not rely on it.

The code is really just:

def _monthlen(year, month):
    return mdays[month] + (month == February and isleap(year))

Which is also basically the same as

calendar.monthrange(year, month)[1]

plus monthrange checks if month is a valid value.

So I would recommend you make your own function that does exactly the same instead of using "private" functions.

You can always check what's available with dir(module) , eg dir(calendar) :

['Calendar',
 'EPOCH',
 'FRIDAY',
 'February',
 'HTMLCalendar',       
 'IllegalMonthError',  
 'IllegalWeekdayError',
 'January',
 'LocaleHTMLCalendar', 
 'LocaleTextCalendar', 
 'MONDAY',
 'SATURDAY',
 'SUNDAY',
 'THURSDAY',
 'TUESDAY',
 'TextCalendar',       
 'WEDNESDAY',
 '_EPOCH_ORD',
 '__all__',
 '__builtins__',       
 '__cached__',
 '__doc__',
 '__file__',
 '__loader__',
 '__name__',
 '__package__',
 '__spec__',
 '_colwidth',
 '_locale',
 '_localized_day',
 '_localized_month',
 '_monthlen',
 '_nextmonth',
 '_prevmonth',
 '_spacing',
 'c',
 'calendar',
 'datetime',
 'day_abbr',
 'day_name',
 'different_locale',
 'error',
 'firstweekday',
 'format',
 'formatstring',
 'isleap',
 'leapdays',
 'main',
 'mdays',
 'month',
 'month_abbr',
 'month_name',
 'monthcalendar',
 'monthrange',
 'prcal',
 'prmonth',
 'prweek',
 'repeat',
 'setfirstweekday',
 'sys',
 'timegm',
 'week',
 'weekday',
 'weekheader']

And ofc there's also the docs: https://docs.python.org/3/library/calendar.html

Answer 2

The calendar should just come with Python. It looks like you are activating your virtual environment correctly. It is not throwing an error that states there is no calendar module it just says that it has no attribute monthlen . My guess is that it might be that the file you are trying to run is called calendar.py or a file around it is named that, so it is importing that file instead of the built-in module.