CODEWITHSUNDEEP
python
Li Haoyi
Li Haoyi

Reputation: 15802

Python: passing flags to functions

For a long time i have been trying to figure out what is the best way to pass flags to python functions. The most straightforward way is something like:

def func(data, flag1, flag2, flag3):
    ...

func(my_data, True, False, True)

This is really nice and concise, but incredibly hard to read, since the word "True" or "False" tells you nothing about what flag is being set, and you have to carefully count the arguments starting from the left. You can make them keyword arguments:

def func(data, flag1=False, flag2=False, flag3=False):
    ...

func(my_data, flag1=True, flag3=True)

but this is kind of redundant, since the "True" doesn't carry any meaning at all. I could pass it as a list:

func(mydata, ['flag1', 'flag3'])

or 

func(mydata, [func.flag1, func.flag3])

But the first feels rather dirty, using strings as flags, and the second is still somewhat repetitive. Ideally i want to say something like:

func(my_data, flag1, flag3)

to pass flags to a function with minimal verbosity and redundancy. Is there any way to do something like this in python?

EDIT: I ended up going with:

func(mydata, flagA=1, flagB=1)

mostly for the reasons mentioned: compile-time checking (versus passing in strings), no namespace pollution (as opposed to using global "ENUM"s) and minimal boilerplate (=1 or =0 is only 2 characters, vs 5 or 6 for =True or =False). It also makes setting default values for the flags very easy:

def func(data, flagA=1, flagB=0, flagC=1):
    ...

which is far more clear and far more easy than jumping through hoops to extract and assign defaults to **kwarg-style flags. The flags are basically statically checked and very clear/clean to write. Now if only I could shave off the last two characters...

Upvotes: 10

Views: 49900

Answers (6)

RickyA
RickyA

Reputation: 16029

I am missing the good old bitwise flags:

a = 1
b = 2
c = 4
d = 8

def func( data, flags ):
    print( ( flags & a) == a )
    print( ( flags & b) == b )
    print( ( flags & c) == c )
    print( ( flags & d) == d )

>>>> func("bla", a|c|d)
>>>> True
>>>> False
>>>> True
>>>> True

Upvotes: 5

6502
6502

Reputation: 114481

What about flipping it over?

flag1, flag2, flag3, flag4, flag5, flag6 = range(6)

def func(enable=[], disable=[],
         enabled_by_default=[flag5, flag6]):
    enabled = set(enabled_by_default + enabled) - set(disabled)
    if flag1 in enabled:
        ...
    if flag2 in enabled:
        ...

func(enable = [flag1, flag2, flag3],
     disable = [flag6])

Upvotes: 3

hamstergene
hamstergene

Reputation: 24439

Some Python standard libraries use this:

re.match(pattern, str, re.MULTILINE | re.IGNORECASE)

You can tweak this approach by using *args:

my.func(a, b, c, my.MULTLINE, my.IGNORECASE)

I would really recommend going with flag1=True:

  • it is readable
  • flag name is checked at compile time (unless **kwargs is used)
  • you can use flag=1 and flag=0 instead of True and False to reduce the noise
  • you can temporarily change LONG_FLAG_NAME_YOU_DONT_REMEMBER=True to False without retyping the long name when you will need to change back

Upvotes: 4

Keith
Keith

Reputation: 43024

I prefer functions without flags. Rather do this:

def func_flag1(arg):
    pass # something useful

def func_flag2(arg):
    pass # something for flag 2.

But "flagX" would actually be something meaningful like "do_X_with_option".

I prefer this because it makes it clearer, you keep the functions simpler (fewer bugs), and you don't have to carry some constants into other modules (in the case of flags actually being some kind of enumeration).

Upvotes: -1

Jakub M.
Jakub M.

Reputation: 33827

you can define flag1 ...flagN as global variables, and define your function with func( *args)

FLAG1 = 1
FLAG2 = 2

def func(*args):
   pass

func(FLAG1, FLAG2)

By defining flags separately, instead of using string, you can avoid typos in flags' names and some headache when debugging

Upvotes: 5

Vincent Savard
Vincent Savard

Reputation: 35927

With *args:

def some_func(data, *args):
    # do something
    return args # this is just to show how it works


>>> print some_func(12, 'test', 'test2', 'test3')
('test', 'test2', 'test3')

This is a good question to understand how *args and **kwargs work : *args and **kwargs?

Upvotes: 3

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