function not working when key value is assigned directly from dictionary to a variable python-CodePudding

So I have been learning python for I guess 10 days or so and I came across some weird behavior by python.

Code snippets and output images below:

    #calculator

def add(n1,n2):
  """Adds two numbers"""
  return n1   n2

def subtract(n1,n2):
  """Subtracts two numbers"""
  return n1 - n2

def multiply(n1,n2):
  """Multiplies two numbers"""
  return n1 * n2

def divide(n1,n2):
  """Divides two numbers"""
  return n1 / n2

#we do not add paranthesis because we want to store the names of functions in the dictionary
#we do not want to assign the function and trigger a call for execution itself. Hence only the name of the function will suffice.

operations = {
' ' : add,
'-': subtract,
'*': multiply,
'/': divide,
}

num1 = int(input("What is this first number you want to enter ?\n"))
num2 = int(input("What is this second number you want to enter ?\n"))

for operation_symbols in operations:
  print(operation_symbols)

operation_symbol = input("Pick a symbol from the list above for your calculations")

answer = operations[operation_symbol]
answer(num1,num2)
print(f"{num1} {operation_symbol} {num2} = {answer}")

When I write this code: my output is the image below: output

However, when I make the following changes

#calculator

def add(n1,n2):
  """Adds two numbers"""
  return n1   n2

def subtract(n1,n2):
  """Subtracts two numbers"""
  return n1 - n2

def multiply(n1,n2):
  """Multiplies two numbers"""
  return n1 * n2

def divide(n1,n2):
  """Divides two numbers"""
  return n1 / n2

#we do not add paranthesis because we want to store the names of functions in the dictionary
#we do not want to assign the function and trigger a call for execution itself. Hence only the name of the function will suffice.

operations = {
' ' : add,
'-': subtract,
'*': multiply,
'/': divide,
}

num1 = int(input("What is this first number you want to enter ?\n"))
num2 = int(input("What is this second number you want to enter ?\n"))

for operation_symbols in operations:
  print(operation_symbols)

operation_symbol = input("Pick a symbol from the list above for your calculations")

operation_function = operations[operation_symbol]
answer = operation_function(num1,num2)
print(f"{num1} {operation_symbol} {num2} = {answer}")

The output I get is the desired one: desired output of calculator from above code snippet

I wanted to know why this happens. I have no idea what is the deal with my code. Thanks and Regards.

CodePudding user response：

In the first code snippet, you are trying to print the reference to the function itself, rather than the result of the function call, answer(num1,num2). Change it as follows and you will get the desired result:

print(f"{num1} {operation_symbol} {num2} = {answer(num1, num2)}")

Alternatively,

result = answer(num1, num2)
print(f"{num1} {operation_symbol} {num2} = {result}")

CodePudding user response：

In the first chunk of non-working code, you set:

answer = operations[operation_symbol]

And in the second chunk, you set:

answer = operation_function(num1,num2)

So, in the first chunk, you set answer to the function itself, and in the second chunk, you set answer to the result of applying the func to the two inputs.

What you're seeing is exactly what you should see. Note that, in the first chunk, you grab and save the function, then on the next line you properly call the function with the two arguments, but you don't save the result.

One way to fix your first chunk would be to change:

answer = operations[operation_symbol]
answer(num1,num2)

to:

answer = operations[operation_symbol](num1,num2)

Happy coding!