Flask - using outputs from a function inside another Function

Question

If I define two functions:

def atesting():
    a = 2
    return a

def btesting():
    b = a+ 3
    return b

But in Flask I get an "Internal Server Error" when running the following if "a" hasn't been defined already. Although if I define "a" outside the app ie say a =2, then it works and I get 5.

app = Flask(__name__)

@app.route('/')
def index():
    results = {}

    a = atesting()

    results = btesting()

    return render_template('index.html', results=results)
if __name__ == '__main__':
    app.run()

Index.html:

<html>

      <h1>{{ results }}</h1>

</html>

But just normally in Python I get 5 when i run this:

a = atesting()
btesting()

Why won't Flask use a = atesting() as an input when computing btesting()?

576i · Accepted Answer · 2020-04-10 21:40:05Z

1

The flask error is correct.

For this usecase there is no global a. (It is set inside the index function, so other functions can't see it)

This is different, if you define a globally in a normal program.

If you want to let function btesting seen the a variable, pass it as a parameter.

app = Flask(__name__)

@app.route('/')
def index():
    results = {}

    a = atesting()

    results = btesting(a)

    return render_template('index.html', results=results)

if __name__ == '__main__':
    app.run()

and

def btesting(a):
    b = a + 3
    return b

edited Apr 10, 2020 at 21:40

answered Apr 10, 2020 at 17:59

576i

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1 Comment

simonsays Over a year ago

Thank's for such a quick response, I was stuck on it for so long and you literally solved it in 2 minutes! Works perfectly, and makes sense.

mikeb · Accepted Answer · 2020-04-10 17:59:52Z

The way you have it written, btesting cannot see a. You need to understand scope. Try this for your functions - note that we are telling btesting the value of a by passing it as an argument:

def atesting():
    a = 2
    return a

def btesting(a):
    b = a+ 3
    return b

Then, call it like this (pass the value to the function)

app = Flask(__name__)

@app.route('/')
def index():
    results = {}

    a = atesting()

    results = btesting(a)

    return render_template('index.html', results=results)
if __name__ == '__main__':
    app.run()

Thanks mike, works like a charm and i understand the concept now as well. Appreciate it!

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