Writing code for function / smiles in python-CodePudding

I wrote the following code, but when I run the test underneath it classifies all the chemicals as "None". I don't understand how to fix this. When I replace "smiles" with "test" and run the test then the output is correct.

def hydrocarbon_group(smiles):
    # your code here:
    if "=" in smiles:
        smiles = "alkene"
        #print(smiles, "is an alkene") 
    else:
        if "1" in smiles:
            smiles = "cycloalkane"
            #print(smiles, "is a cycloalkane")
        else:
            if "#" in smiles:
                smiles = "alkyne"
                #print(smiles, "is an alkyne")
            else:
                if "C" in smiles:
                    smiles = "alkane"
                    #print(smiles, "is a alkane")
                else:
                    smiles = "None"
                    #print(smiles, "is none")
        
#test = "C"
#hydrocarbon_group(test)


# test your function by checking whether the output below is correct
print('methane is', hydrocarbon_group("C"))
print('propene is', hydrocarbon_group("C=CC"))
print('ethyne is', hydrocarbon_group('C#C'))
print('cyclobutane is', hydrocarbon_group('C#C'))
print('methyl isocyanate is', hydrocarbon_group('CN=C=O'))

CodePudding user response：

insert return smiles at the bottom of the function it should work.

CodePudding user response：

Here's a slightly different way to do what you're asking. It uses two steps as follows:

In a list comprehension (essentially a one-line loop that creates a list of results for each iteration), record the result of a search for each of your key characters '=1#C' in smiles, and use the first successful search by accessing the element of the search results at index 0 (append a different character such as ' ' to the search results to cover the possibility that smiles is either empty or does not contain any of the key characters).
Using Python's match statement and case blocks, test for each of the key characters sequentially until the first match.

def hydrocarbon_group(smiles):        
    x = ''.join([c[:c in smiles] for c in '=1#C'] [' '])[0]
    match x:
        case "=": 
            return "alkene"
        case "1": 
            return "cycloalkane"
        case "#": 
            return "alkyne"
        case "C": 
            return "alkane"
        case _: 
            return None

print('methane is', hydrocarbon_group("C"))
print('propene is', hydrocarbon_group("C=CC"))
print('ethyne is', hydrocarbon_group('C#C'))
print('cyclobutane is', hydrocarbon_group('C#C'))
print('methyl isocyanate is', hydrocarbon_group('CN=C=O'))

Output:

methane is alkane
propene is alkene
ethyne is alkyne
cyclobutane is alkyne
methyl isocyanate is alkene

The reasons for preferring this approach to the one in your original code are:

Does not rely on nested if/else blocks.
match/case syntax provides a concise, readable expression of what conclusion to draw based on which key character was encountered.
For someone who is new to Python, the "exercise" of trying out the match statement helps build new language skills.

Arguments against this approach could include:

The list comprehension for detecting the first key character encountered in the desired sequence is too cryptic.
if/elif/else would provide an alternative that is also readable (though not as concise as the match statement) and does not require the list comprehension.