如何在python中测试变量为空

Question

val = ""

del val

if val is None:
    print("null")

我运行了上面的代码，但是得到了NameError: name 'val' is not defined。

如何判断变量是否为空，避免命名错误？

Answer 1

try:
    if val is None: # The variable
        print('It is None')
except NameError:
    print ("This variable is not defined")
else:
    print ("It is defined and has a value")

Answer 2

测试指向None的名称和现有名称是两个语义不同的操作。

要检查val是否为None：

if val is None:
    pass  # val exists and is None

要检查名称是否存在：

try:
    val
except NameError:
    pass  # val does not exist at all

Answer 3

您可以在try-and-catch块中执行此操作：

try:
    if val is None:
        print("null")
except NameError:
    # throw an exception or do something else