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python course 1 week 5 用缩进，不是花括号 Nested If if-else Multi-way (“elif” which is “else if”) try-except (catch errors)

admin 11月 13, 2020 0

Conditional statements (if, elif, else) in Python 3.

x = 5
if x < 10:
    print('Smaller')
if x > 20:
    print ('Larger')
print('Finish')

用缩进，不是花括号

if x > 2:
    print('Bigger than 2') 
    print('Still bigger') #still in the block
print('Done with 2') #out of the if

for i in range(5): #loop
    print(i)
    if i > 3:
        print('Bigger than 3')
    print('Done with i', i); #out of if
print('All done') #out of the for loop

Nested If

y = 42
if y > 1:
    print('Y is larger than 1')
    if y < 100:
        print('Y is smaller than 100')
print('All done')

if-else

y = 100
if y == 100:
    print('Y is 100')
else:
    print('Y is not 100')

Multi-way (“elif” which is “else if”)

if y > 100:
    print('Y is larger than 100')
elif y == 100:
    print('Y is 100')
else:
    print('Y is smaller than 100')

try-except (catch errors)

str1 = 'Hello'
try:
    num1 = int(str1) # trackback, error here
except:
    num1 = -1 # this return
print('first string-', num1) # output: first string- -1

str2 = '123'
try:
    num1 = int(str2) # works
except:
    num1 = -1
print('second string-', num1) # output: second string- 123

图中1正常，2出错，到3，4是不会运行的。所以在try里面写过多逻辑是不合理的，尽量简单。