Trying to evaluate user input and use it as a variable in python

Question

I am a beginner to python and any coding at all for that matter, and I am having some trouble with some code I have written:

loop = 0
hellosentence = "You have selected the hello sentence"
questionsentence = "You have selected the question sentence"

usrinput  = raw_input("Which test would you like to bring up? ")

print eval(usrinput)+sentence

end = raw_input("Press any key to quit...")

I am trying to run the code with this in mind.

Have multiple names with sentence at the end.

Request which "sentence" the user wants to bring up

Once the user has typed in the sentence, it evaluates what they have typed, say "hello" and combines it with "sentence" to make the variable "hellosentence", thus bringing up the variable's string.

I'm 14 and really trying to learn how to understand this language, so if anyone knows what I could do to fix this error, I would be very grateful.

Keep data out of your variable names. Use a dictionary instead. — Sven Marnach
– Sven Marnach, Commented Feb 28, 2012 at 22:13
never mind I found that making a variable sentence = "sentence" and evaluate that with the string usrinput has the desired output. — crazedsuirlz
– crazedsuirlz, Commented Feb 28, 2012 at 22:36

Mark Ransom · Accepted Answer · 2012-02-28 22:46:38Z

3

eval is going to take the string you give it and evaluate it. In this case it's taking the input string that was typed in. Judging by the variables you've defined, you want to take the input and append sentence to it, so if they type 'hello' you'll return the value of hellosentence.

The direct way to fix it is to give eval the proper input:

print eval(usrinput+'sentence')

That probably solves your problem, but it's not a good way to do it. eval is the wrong tool for the job; error recovery is going to be a problem, and if this were production code you'd be opening a big hole for someone who wanted to turn your program into a virus.

The proper tool is a dictionary, with which you can use the input to look up the proper output.

selection = {'hello': hellosentence, 'question': questionsentence}
print selection[usrinput]

answered Feb 28, 2012 at 22:46

Mark Ransom

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1 Comment

crazedsuirlz Over a year ago

That is perfect. I totally understand now. I guess I never really understood how to use dictionaries. I'll go back and do a tutorial on them again. Thank you so much.

purpleladydragons · Accepted Answer · 2012-02-28 22:29:10Z

First of all, your

loop = 0

line is pointless. You can see it is never used again.

print eval(usrinput)+sentence

I can see what you're trying to accomplish, printing hellosentence as in the variable. It does not work that way however. Using eval() doesn't do much of anything for you, so imagine writing

print usrinput+sentence

This wouldn't print the value of whatever variable has the same beginning as usrinput and an ending of sentence. It would print the value of usrinput and then print the value of sentence. Since you do not have a sentence variable, you are getting an error because the value of sentence does not exist.

Say the user entered "hello" when prompted for his input. The print line would not print the value of hellosentence, but would print "hello", and then have an error because sentence does not exist. Basically, you can't treat this as pure English because you can have something like this:

pretzel = "apple"
print pretzel

This will print the word "apple" and not "pretzel". If you do not understand this, I highly recommend you read about variables and assignment.

You have to check the value of usrinput.

if usrinput = "hello":
    print hellosentence

Your end variable at the last line of your program does nothing, as you just assign the value to end. Try checking the value of end as we did with usrinput.

It seems that you have a misunderstanding about how the program actually executes. I would recommend reading about variables and assignment.

Aprillion · Accepted Answer · 2012-02-28 23:02:05Z

0

this should get the job done - anything except the insecure parts of eval ;))

input = raw_input # remove this if you already use Python 3...

def my_func():
    for i in range(10):
        pass
    return "How are you today?"

sentences = {}
sentences["hello"] = "Hello world..."
sentences["question"] = my_func

usrinput = input("Which test...?")

try:
    obj = sentences[usrinput]
    if hasattr(obj, '__call__'):
        print(obj())
    else:
        print(obj)
except KeyError:
    print("Invalid input...")
finally:
    input("Press ENTER to quit...")

answered Feb 28, 2012 at 23:02

Aprillion

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