Python/输入输出
< Python
键盘输入
编辑内置函数input()从标准输入(sys.stdin)读入一行文本,返回值要去掉回车符。抛出EOFError如果过早结束。
使用函数 eval() 把input()返回的值解释执行。
文件
编辑返回文件对象:
open(filename, mode)
文件对象的方法:
- close()
- fileno()
- flush()
- isatty()
- next()
- read(size)
- readline()
- readlines() #也可以 for line in f:
- seek()
- tell()
- truncate([size])
- write(string)
- writelines(sequence)
Python 3的read/write函数默认只接受string类型。二进制字节流,必须用'wb'这样的模式打开文件。
pickle模块实现了数据序列化和反序列化。方法为dump与load
Standard File Objects
编辑import sys
for line in sys.stdin:
print (line,)
sys.stdin.read()读到EOF为止(通常为Ctrl+D)
分析命令行
编辑sys.argv是一个列表,可修改。第一个元素是程序名本身(是否全路径不一定)。
import sys
for arg in sys.argv[1:]:
print (arg)
分析减号命名的命令行选项:
import sys
option_f = False
option_p = False
option_p_argument = ""
i = 1
while i < len(sys.argv):
if sys.argv[i] == "-f":
option_f = True
sys.argv.pop(i)
elif sys.argv[i] == "-p":
option_p = True
sys.argv.pop(i)
option_p_argument = sys.argv.pop(i)
else:
i += 1
分析命令行还可以用包argparse (从Python 2.7) 与getopt (C程序员).
输出
编辑输出值的方式:
- print() 函数。
- 文件对象的 write() 方法,标准输出文件可以用 sys.stdout 引用。
- str.format() 函数来格式化输出值。
- str(): 函数返回一个用户易读的表达形式。
- repr(): 产生一个解释器易读的表达形式。
任何值都可以print。对于元组打印效果是在各项之间加一个空格来区隔:
print 1,2,0xff,0777,(10+5j),-0.999,map,sys
输出为:
1 2 255 511 (10+5j) -0.999 <built-in function map> <module 'sys' (built-in)>
for i in range(10):
print i,
输出:
0 1 2 3 4 5 6 7 8 9
打印换行符的办法是打印一个空行:
for i in range(10):
print i,
print
for i in range(10,20):
print i,
输出
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
写到文件:
print >> f, 'Hello, world'
任何实现了write()的对象都可以如上做。
忽略空格与换行符
编辑为了避免在输出结果中增加空格与换行符,有几个办法:
连接:
print str(1)+str(2)+str(0xff)+str(0777)+str(10+5j)+str(-0.999)+str(map)+str(sys)
输出
12255511(10+5j)-0.999<built-in function map><module 'sys' (built-in)>
Write函数,即sys.stdout.write:
import sys
write = sys.stdout.write
write('20')
write('05\n')
输出
2005
用sys.stdout.flush()刷到屏幕
Python 3.x的示例:
- from __future__ import print_function
- Ensures Python 2.6 and later Python 2.x can use Python 3.x print function.
- print ("Hello", "world")
- Prints the two words separated with a space. Notice the surrounding brackets, ununsed in Python 2.x.
- print ("Hello world", end="")
- Prints without the ending newline.
- print ("Hello", "world", sep="-")
- Prints the two words separated with a dash.
- print ("Error", file=sys.stderr)
- Outputs to a file handle, in this case standard error stream.
文件输出
编辑用换行符分隔开:
file1 = open("TestFile.txt","w")
for i in range(1,10+1):
print >>file1, i
file1.close()
使用减号分隔开,没有换行符:
file1 = open("TestFile.txt","w")
for i in range(1,10+1):
if i>1:
file1.write("-")
file1.write(str(i))
file1.close()
追加:
file1 = open("TestFile.txt","a")
格式化
编辑格式化到字符串,使用“字符串百分号运算符”:
v1 = "Int: %i" % 4 # 4
v2 = "Int zero padded: %03i" % 4 # 004
v3 = "Int space padded: %3i" % 4 # 4
v4 = "Hex: %x" % 31 # 1f
v5 = "Hex 2: %X" % 31 # 1F - capitalized F
v6 = "Oct: %o" % 8 # 10
v7 = "Float: %f" % 2.4 # 2.400000
v8 = "Float: %.2f" % 2.4 # 2.40
v9 = "Float in exp: %e" % 2.4 # 2.400000e+00
vA = "Float in exp: %E" % 2.4 # 2.400000E+00
vB = "List as string: %s" % [1, 2, 3]
vC = "Left padded str: %10s" % "cat"
vD = "Right padded str: %-10s" % "cat"
vE = "Truncated str: %.2s" % "cat"
vF = "Dict value str: %(age)s" % {"age": 20}
vG = "Char: %c" % 65 # A
vH = "Char: %c" % "A" # A
格式化到字符串,使用format()字符串方法,Python 2.6引入:
v1 = "Arg 0: {0}".format(31) # 31
v2 = "Args 0 and 1: {0}, {1}".format(31, 65)
v3 = "Args 0 and 1: {}, {}".format(31, 65)
v4 = "Arg indexed: {0[0]}".format(["e1", "e2"])
v5 = "Arg named: {a}".format(a=31)
v6 = "Hex: {0:x}".format(31) # 1f
v7 = "Hex: {:x}".format(31) # 1f - arg 0 is implied
v8 = "Char: {0:c}".format(65) # A
v9 = "Hex: {:{h}}".format(31, h="x") # 1f - nested evaluation
格式化到字符串,使用字面量字符串插值, Python 3.6引入:
int1 = 31; int2 = 41; str1="aaa"; myhex = "x"
v1 = f"Two ints: {int1} {int2}"
v2 = f"Int plus 1: {int1+1}" # 32 - expression evaluation
v3 = f"Str len: {len(str1)}" # 3 - expression evaluation
v4 = f"Hex: {int1:x}" # 1f
v5 = f"Hex: {int1:{myhex}}" # 1f - nested evaluation