當鍵值直接從字典分配給變量 python 時，函數不起作用

Question

所以我一直在學習 python 大約 10 天左右，我遇到了 python 的一些奇怪的行為。

下面的代碼片段和輸出圖像：

    #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}")

當我寫這段代碼時：我的輸出是下圖：輸出

但是，當我進行以下更改時

#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}")

我得到的輸出是想要的：上面代碼片段的計算器的期望輸出

我想知道為什么會發生這種情況。 我不知道我的代碼是怎么回事。 感謝致敬。

Answer 1

在第一個代碼片段中，您嘗試打印對函數本身的引用，而不是函數調用的結果answer(num1,num2) 。 按如下方式更改它，您將獲得所需的結果：

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

或者，

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

Answer 2

在第一塊非工作代碼中，您設置：

answer = operations[operation_symbol]

在第二個塊中，您設置：

answer = operation_function(num1,num2)

因此，在第一個塊中，將answer設置為函數本身，在第二個塊中，將answer設置為將 func 應用於兩個輸入的結果。

你所看到的正是你應該看到的。 請注意，在第一個塊中，您獲取並保存該函數，然后在下一行使用兩個參數正確調用該函數，但不保存結果。

修復第一個塊的一種方法是更改​​：

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

到：

answer = operations[operation_symbol](num1,num2)