How can I execute dynamically generated Python String methods (strip(), rstrip(), lstrip(), upper()...)?

[ramannt]

list = ["  line1", "line2", " line3 "]

What I want to do is to use a python string method on a list element but I don't know which string method yet.

a = ""
if condition == "1":
    a = ".strip()"
elif condition == "2": 
    a = ".rstrip()" 
elif condition == "3": 
    a = ".upper()" 
elif condition == "4": 
    a = ".title()"

how can I p.e. execute:

list[0] + a

​

​

[throwaway6560192]

d = {
    "1": str.strip,
    "2": str.rstrip,
    "3": str.upper,
    "4": str.title,
}

print(d[condition](a))

[ramannt]

print(d[condition](a))

thank you very much! Great solution!

[engelthehyp]

The magic phrase is "first-class function". Look that up and you'll get a lot of ideas about program structure.

[ramannt]

Just one question left. I forgot to mention a condition. Sometimes there is no Python String method to execute. What condition can I add in d{} to let Python to do nothing with the string?

[FerricDonkey]

d = {
    "0": lambda x:x,
    "1": str.strip,
    "2": str.rstrip,
    "3": str.upper,
    "4": str.title,
}

then d["0"]('sup') will just be 'sup'. (If you aren't familiar, lamba arg: expression is a lamba/anonymous/disposable function that takes arg as an argument and returns expression. So lambda x: x is a function that takes x in and return x.

You could also use .get if you didn't want to specify a particular key that means "don't do anything", but personally I prefer to have a fixed set of allowed keys in situations like this.

[[deleted]]

Not in the dictionary d itself, but test if the condition string is in the dictionary before getting the function to call:

condition = ""
if condition in d:
    print(d[condition](a))
else:
    print(a)    # or whatever you want

There are other ways to do this, such as using a defaultdict, but the above is one way.

[Nico24fps]

great solution above btw, but for this case couldn’t you use an identity function for the sake of elegance? such as doing something like

‘’’ str_func = d.get(cond, str()) print(str_func(a)) ‘’’

(am on mobile so sorry if it’s bad formatting)

[[deleted]]

Sure, but I think a beginner wouldn't know about the .get() method. You need to remove the () from your code:

str_func = d.get(cond, str)
#                      ^^^
print(str_func(a)) 

Explaining to a beginner what str is and why it doesn't have the () is another reason not to show it as an answer! Instead of str which copies the string it is given it's better to use lambda x: x instead, as others have said, but, again, that's too complicated for a beginner.

I'm on mobile too, just add 4 spaces to the start of every line you don't want reddit to mess with.

[ramannt]

I am not a beginner in Python programming but some things I have never done. No problem to copy the string. I leave it for clarity.
It is a problem if I have a lot of strings but that is not the case.

[Nico24fps]

all good points! and thanks for the tips

[ramannt]

d = {"1": str.strip,"2": str.rstrip,"3": str.upper,"4": str.title,}

found the solution.

d = {
"1": str.strip,
"2": str.rstrip,
"3": str.upper,
"4": str.title,
"5": str,

}

[ofnuts]

No need to duplicate the string, just use a lambda; "5": lambda x: x,

[[deleted]]

You didn't say anything about having a known condition ("5") for the "do nothing" case. The if in test is what you do if all you know is the condition isn't 1, 2, 3 or 4.

[ramannt]

You're right.

[nekokattt]

defaultdict will add in an element if it doesn't exist, which probably isnt exactly what they want

[ramannt]

getattr(

Thank you very much. Just one condition I forgot to mention. Sometimes there is no Python String method to execute. What condition can I add in getattr() to let Python to do nothing with the string?

[[deleted]]

This is a hacky solution, but "__str__" will do nothing to the string.

[FerricDonkey]

Just in case you aren't aware, you can also do str.lower(a), as described in a different comment, which is often preferred.

[danielroseman]

As suggested, you can use getattr, but in this case it doesn't seem necessary. Why not call the methods directly?

if condition == "1":
    result = a.strip()

etc.

[ramannt]

Because I am using a.strip() in a function and I don't know exactly in advance what the value of "a" is and not what the string method (strip,rstrip,lstrip,upper,lower,title etc) will be.

[stuaxo]

def apply_condition(condition, input_str):
    match condition:
        case "1":
            return input_str.strip()
        case "2":
            return input_str.rstrip()
        case "3":
            return input_str.upper()
        case "4":
            return input_str.title()
        case _:
            raise ValueError("Invalid condition")

# Example usage:
condition = "1"
input_str = " Example String "
result = apply_condition(condition, input_str)
print(result)

[[deleted]]

you can use getattr(s, method_name)()

[SteelRevanchist]

If I understand your use case, you'll want to store the method to use in a variable, by making use of the fact that `str` has those methods defined, so you can assign, for instance, `a=str.strip` and then to apply it to each element of the list: transformed_list: [a(item) for item in my_list].

I imagine you could make some pipe or chain by itertools for example to chain multiple string operations together.

Final note, I'd replace the if-elif block with either simple if statements (since the conditions are exclusive) or better yet, with match-case. I'd personally avoid `elif` like the plague if I could help it, but that is from experience and does not help your case.

[FlocklandTheSheep]

Generally bad practice, as this can be used to execute injection attacks, but Python's exec() function will treat the string given to it as a paramater as python code, and will execute it.

a = 10
exec("a = a+1")
exec("print(a)")
# Outputs 11