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条件
Python 使用布尔逻辑来评估条件。当一个表达式被比较或评估时，会返回布尔值 True 和 False。例如：
python
x = 2
print(x == 2) # 输出 True
print(x == 3) # 输出 False
print(x < 3) # 输出 True
注意，变量赋值用单个等号“=”，而两个变量的比较则用双等号“==”。“不等于”运算符表示为“!=”。
布尔运算符
“and”和“or”布尔运算符允许构建复杂的布尔表达式，例如：
```python name = "John" age = 23 if name == "John" and age == 23: print("Your name is John, and you are also 23 years old.")
if name == "John" or name == "Rick": print("Your name is either John or Rick.") ```
"in" 运算符
“in”运算符可以用来检查指定对象是否存在于可迭代对象容器中，例如列表：
python
name = "John"
if name in ["John", "Rick"]:
print("Your name is either John or Rick.")
Python 使用缩进来定义代码块，而不是括号。标准的 Python 缩进是 4 个空格，虽然制表符和其他任意数量的空格也可以，只要保持一致即可。注意，代码块不需要任何终止符。
这是一个使用 Python "if" 语句的代码块示例：
python
statement = False
another_statement = True
if statement is True:
# 做某事
pass
elif another_statement is True: # 否则如果
# 做其他事
pass
else:
# 做另一件事
pass
例如：
python
x = 2
if x == 2:
print("x equals two!")
else:
print("x does not equal to two.")
如果以下其中之一正确，则语句被评估为真： 1. 给定了布尔变量 "True"，或者通过表达式计算得出，例如算术比较。 2. 传递了一个不被认为是“空”的对象。
以下是一些被认为是空的对象示例： 1. 空字符串："" 2. 空列表：[] 3. 数字零：0 4. 布尔变量 False: False
'is' 运算符
与双等号运算符“==”不同，“is”运算符不匹配变量的值，而是匹配实例本身。例如：
python
x = [1,2,3]
y = [1,2,3]
print(x == y) # 输出 True
print(x is y) # 输出 False
"not" 运算符
在布尔表达式前使用“not”会反转它：
python
print(not False) # 输出 True
print((not False) == (False)) # 输出 False
Exercise
更改第一部分的变量，使每个 if 语句的结果为 True。
# change this code
number = 10
second_number = 10
first_array = []
second_array = [1,2,3]
if number > 15:
print("1")
if first_array:
print("2")
if len(second_array) == 2:
print("3")
if len(first_array) + len(second_array) == 5:
print("4")
if first_array and first_array[0] == 1:
print("5")
if not second_number:
print("6")
# change this code
number = 16
second_number = 0
first_array = [1,2,3]
second_array = [1,2]
if number > 15:
print("1")
if first_array:
print("2")
if len(second_array) == 2:
print("3")
if len(first_array) + len(second_array) == 5:
print("4")
if first_array and first_array[0] == 1:
print("5")
if not second_number:
print("6")
test_output_contains("1", no_output_msg= "Did you print out 1 if `number` is greater than 15?")
test_output_contains("2", no_output_msg= "Did you print out 2 if there exists a list `first_array`?")
test_output_contains("3", no_output_msg= "Did you print out 3 if the length of `second_array` is 2?")
test_output_contains("4", no_output_msg= "Did you print out 4 if len(first_array) + len(second_array) == 5?")
test_output_contains("5", no_output_msg= "Did you print out 5 if first_array and first_array[0] == 1?")
test_output_contains("6", no_output_msg= "Did you print out 6 if not second_number?")
success_msg("Great Work!")
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