Python pass second argument in method call based on if check

Function/Methods are used to pass arguments based on their declaration and usage. Evaluation or doing any operation during declaration or usage is a bad approach and design.

What's the harm in using var-positional arguments i.e *args and *kwargs? The code should be simplified and not complicate. On top of that, think in terms of Unit testing your code function/code? Or may be you're not interested to unit test. Also I would recommend to go through PEP-0362

You can do your evaluation in if condition and then pass it as argument. In your case you could simply do like below:

Your function declaration:

def get_quest_guide_char(name: str, **kwargs):
    if 'marker_sector' in kwargs:
        market_sector = kwargs.get('marker_sector')

If you need to pass multiple other parameters, you can iterate over the kwargs dict:

def get_quest_guide_char(name: str, **kwargs):
    for key, value in kwargs.items():
        if key == 'marker_sector':
           market_sector = value

Usage:

 if value == "school_english_class":
        get_quest_guide_char(name="isabelle", marker_sector="right_bottom")
    else:
        get_quest_guide_char(name="isabelle")

NOTE: I used named parameter for value 'isabelle' to make things easier, you can choose to omit or use.

You can do

get_quest_guide_char("isabelle", marker_sector="right_bottom" if game.location == "school_english_class" else None))

This will pass to the function "right_bottom" only if game.location == "school_english_class" else it will pass None and then handle None in the function.
If you function defined as get_quest_guide_char(isabelle_value, marker_sector) then you can use just "right_bottom" if game.location == "school_english_class" else None without the marker_sector=.

It's probably better to just build your extra parameter outside of the method call, it makes it easier to read specially if the method parameters starts increasing.

params = {}
if game.location == "school_english_class":
    params['mark_sector'] = "right_bottom"

get_quest_guide_char("isabelle", **params)

Like the other answers before mine, this technically still passes at least two arguments. This is because function overloading isn't really a thing in Python, (having multiple functions with the same name). If you want a variable number of parameters, you can use *args and/or **kwargs when calling the function, but your function would still technically have to be defined with multiple arguments:

def get_quest_guide_char(character, *args, **kwargs):
    # ...

# or in your case
def get_quest_guide_char(character, *args):
    # ...

Or more explicitly:

def get_quest_guide_char(character, marker_sector=None):
    # ...

This will let you do both:

get_quest_guide_char("isabelle", marker_sector)

# and
get_quest_guide_char("isabelle")

Given that marker_sector depends on game.location, you could alternatively create another function that gives you the marker_sector, and simply pass that into your function regardless:

def get_marker_sector(location):
    if (location == "school_english_class"):
        marker_sector = "right_bottom"
    else:
        # ...

    return marker_sector

marker_sector = get_marker_sector(game.location)
get_quest_guide_char("isabelle", marker_sector):

Alternatively, you could just pass in game.location to your get_quest_guide_char function:

def get_quest_guide_char(character, location):
    if (location == "school_english_class"):
        # ...
    else:
        # ...

get_quest_guide_char("isabelle", game.location)

Either way, I hope you're not intending on using marker_sector as a global variable inside any of your functions. Mutable global variables are asking for trouble.