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# for continue 举例
for l in 'Python':
if l == 'h':
continue
print('letter:', l)
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# for continue 举例
# 去除输入的内容中的元音字母
list1 = input('Input:')
list2 = []
for s in list1:
if s in ['a','e','i','o','u']:
continue
list2.append(s)
# 将列表转换为字符串
s2 = ''.join(list2)
print('Output:', s2)
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# for break else 举例
# 判断质数
import time
a = time.time()
for num in range(3,20):
for i in range(2,num):
if num%i == 0:
j=num/i
print(num,i,'*',j)
break
else:
print(num, '是质数')
b = time.time()
print(b-a)
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import sys
print(sys.path)
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import sys
import pprint
pprint.pprint(sys.path)
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# jupyter notebook 中直接显示一些变量的结果就和采用了 pprint 的方法
import sys
sys.path
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import sys
print(type(sys.path))
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# 简单四则计算器代码，下面代码参考 https://www.jianshu.com/p/c22bfd91df49
def calculate():
operation = input('''
please type in the math operation you would like to complete:
+ for addition
- for subtraction
* for multiplication
/ for division
''')
number_1 = float(input('Please enter the first number: '))
number_2 = float(input('Please enter the second number: '))
if operation == '+':
print('{} + {} = {} '.format(number_1, number_2,number_1 + number_2))
elif operation == '-':
print('{} - {} = {} '.format(number_1,number_2,number_1 - number_2))
elif operation == '*':
print('{} * {} = {} '.format(number_1, number_2,number_1 * number_2))
elif operation == '/' and number_2 != 0:
print('{} / {} = {} '.format(number_1, number_2,number_1 / number_2))
else:
print('You have not typed a valid operator, please run the program again.')
calculate()
如果你对使用 python 来构造一种语言，比如适用一些场景的 DSL 感兴趣，如果你觉得 lex 和 yacc 这些编译原理中的概念感兴趣的话，可以参考这里：https://www.dabeaz.com/software.html
Python 下有很多优秀的包，都可以来实现语言定义和解析，但是 ply 是其中最经典的包，从2001年诞生至今，一致在维护更新，并且不断发展。
下面这个是 ply 自己带的例子，用来实现一个比较完整的计算器。因为是通过语法分析的方式实现，会比较费解，但是功能强大，并且容易扩充。
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# -----------------------------------------------------------------------------
# calc.py
#
# A simple calculator with variables -- all in one file.
# -----------------------------------------------------------------------------
tokens = (
'NAME', 'NUMBER',
'PLUS', 'MINUS', 'TIMES', 'DIVIDE', 'EQUALS',
'LPAREN', 'RPAREN',
)
# Tokens
t_PLUS = r'\+'
t_MINUS = r'-'
t_TIMES = r'\*'
t_DIVIDE = r'/'
t_EQUALS = r'='
t_LPAREN = r'\('
t_RPAREN = r'\)'
t_NAME = r'[a-zA-Z_][a-zA-Z0-9_]*'
def t_NUMBER(t):
r'\d+'
try:
t.value = int(t.value)
except ValueError:
print("Integer value too large %d", t.value)
t.value = 0
return t
# Ignored characters
t_ignore = " \t"
def t_newline(t):
r'\n+'
t.lexer.lineno += t.value.count("\n")
def t_error(t):
print("Illegal character '%s'" % t.value[0])
t.lexer.skip(1)
# Build the lexer
import ply.lex as lex
lexer = lex.lex()
# Parsing rules
precedence = (
('left', 'PLUS', 'MINUS'),
('left', 'TIMES', 'DIVIDE'),
('right', 'UMINUS'),
)
# dictionary of names
names = {}
def p_statement_assign(t):
'statement : NAME EQUALS expression'
names[t[1]] = t[3]
def p_statement_expr(t):
'statement : expression'
print(t[1])
def p_expression_binop(t):
'''expression : expression PLUS expression
| expression MINUS expression
| expression TIMES expression
| expression DIVIDE expression'''
if t[2] == '+':
t[0] = t[1] + t[3]
elif t[2] == '-':
t[0] = t[1] - t[3]
elif t[2] == '*':
t[0] = t[1] * t[3]
elif t[2] == '/':
t[0] = t[1] / t[3]
def p_expression_uminus(t):
'expression : MINUS expression %prec UMINUS'
t[0] = -t[2]
def p_expression_group(t):
'expression : LPAREN expression RPAREN'
t[0] = t[2]
def p_expression_number(t):
'expression : NUMBER'
t[0] = t[1]
def p_expression_name(t):
'expression : NAME'
try:
t[0] = names[t[1]]
except LookupError:
print("Undefined name '%s'" % t[1])
t[0] = 0
def p_error(t):
print("Syntax error at '%s'" % t.value)
import ply.yacc as yacc
parser = yacc.yacc()
while True:
try:
s = input('calc > ') # Use raw_input on Python 2
except EOFError:
break
parser.parse(s)
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# 不能直接在 notebook 中执行上面代码，%run 命令执行 python 脚本
# 执行上面的计算器代码，支持加减乘除和括号，支持变量，并且具备非常好的扩展性
%run "calc.py"
# 1+1*2
# (1+1)*2
# (((2+3)*41)-8)+78
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