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本文目录一览：
1、为什么Python中print(eval(input(3+4*5)))输出为23？2、python函数返回值为”id“：“23”，如何获取233、python运算符为什么Python中print(eval(input(3+4*5)))输出为23？
一步一步分解：
1 计算Input(3+4*5)得到的是：'3+4*5'
2 使用eval可以计算字符表达式的值，所以是3+4*5=3+20=23
python函数返回值为”id“：“23”，如何获取23
最近遇到os.system()执行系统命令的情况，上网搜集了一下资料，整理如下，以备不时之需，同时也希望能帮到某些人。
一、python中的 os.system(cmd)的返回值与linux命令返回值（具体参见本文附加内容）的关系
大家都习惯用os.systemv()函数执行linux命令，该函数的返回值十进制数（分别对应一个16位的二进制数）。该函数的返回值与 linux命令返回值两者的转换关系为：该函数的返回值（十进制）转化成16二进制数，截取其高八位（如果低位数是0的情况下，有关操作系统的错误码共 131个，所以低位都是零），然后转乘十进制数即为 linux命令返回值0。
例如：
os.system()返回值为0 linux命令返回值也为0.
os.system()返回值为256，十六位二进制数示为：00000001，00000000，高八位转乘十进制为 1 对应 linux命令返回值 1
os.system()返回值为512，十六位二进制数示为：00000010，00000000，高八位转乘十进制为 2 对应 linux命令返回值 2
......其它同理
os.system()返回值为32512，十六位二进制数示为：01111111，00000000，高八位转乘十进制为 127 对应 linux命令返回值 127
........
/**********************************************************************************************************************/
问题：/bin/xxx.py是一个返回码为1的程序。当python 程序使用os.system(”./bin/xxx.py”) 这样调用的时候， 成功运行后os.system 的返回值出现了问题，变成了256 ，也就是0×100。而不是正常应该返回的1。
解决：查阅了文档发现os.system()的返回为：
On Unix, the return value is the exit status of the process encoded in the format specified for wait().
而os.wait()的返回为：
a 16-bit number, whose low byte is the signal number that killed the process, and whose high byte is the exit status (if the signal number
is zero);
os.system的返回值并不是执行程序的返回结果。而是一个16位的数，它的高位才是返回码。也就是说os.system()返回256即 0×0100，返回码应该是其高位0×01即1。所以要获取程序运行退出的值（比如C的main函数中的return 0），需要处理一下。
ret = os.system('./a.out')
ret = 8
这样才能获取到正确的返回值。另外还要注意：python获取到的值是无符号整数，所以返回负值的时候，打印出来是很大的正值。比如返回-1，python 会获取到255，-2则254，以此类推。所以最好就判断是否为0就可以了，实在要判断自己写的c程序返回值，建议返回0,1,2,3等值，出错返回 -1。
另外，我遇到一次明明处理好了返回值，c程序调试信息提示也该返回值0了，结果python获取到的是 -1，而且无论c程序返回多少，python都获取-1。后来排查c程序的问题，发现原来是因为我这个python程序本身是由另一个C程序调用的，而调 用它的那个C程序中将SIGCLD信号忽略了（这表明python是根据子进程退出时产生的信号来获取返回值的），我将那个C程序的SIGCLD绑定到函 数，即使那个函数什么也不做，python也能获取到正确的返回值了。
/**********************************************************************************************************************/
linux命令执行后无论成功与否都有一个返回值：
如果为 0，则表示命令执行成功，其它值则表示错误，具体的错误码含义如下：
"OS error code 1: Operation not permitted"
"OS error code 2: No such file or directory"
"OS error code 3: No such process"
"OS error code 4: Interrupted system call"
"OS error code 5: Input/output error"
"OS error code 6: No such device or address"
"OS error code 7: Argument list too long"
"OS error code 8: Exec format error"
"OS error code 9: Bad file descriptor"
"OS error code 10: No child processes"
"OS error code 11: Resource temporarily unavailable"
"OS error code 12: Cannot allocate memory"
"OS error code 13: Permission denied"
"OS error code 14: Bad address"
"OS error code 15: Block device required"
"OS error code 16: Device or resource busy"
"OS error code 17: File exists"
"OS error code 18: Invalid cross-device link"
"OS error code 19: No such device"
"OS error code 20: Not a directory"
"OS error code 21: Is a directory"
"OS error code 22: Invalid argument"
"OS error code 23: Too many open files in system"
"OS error code 24: Too many open files"
"OS error code 25: Inappropriate ioctl for device"
"OS error code 26: Text file busy"
"OS error code 27: File too large"
"OS error code 28: No space left on device"
"OS error code 29: Illegal seek"
"OS error code 30: Read-only file system"
"OS error code 31: Too many links"
"OS error code 32: Broken pipe"
"OS error code 33: Numerical argument out of domain"
"OS error code 34: Numerical result out of range"
"OS error code 35: Resource deadlock avoided"
"OS error code 36: File name too long"
"OS error code 37: No locks available"
"OS error code 38: Function not implemented"
"OS error code 39: Directory not empty"
"OS error code 40: Too many levels of symbolic links"
"OS error code 42: No message of desired type"
"OS error code 43: Identifier removed"
"OS error code 44: Channel number out of range"
"OS error code 45: Level 2 not synchronized"
"OS error code 46: Level 3 halted"
"OS error code 47: Level 3 reset"
"OS error code 48: Link number out of range"
"OS error code 49: Protocol driver not attached"
"OS error code 50: No CSI structure available"
"OS error code 51: Level 2 halted"
"OS error code 52: Invalid exchange"
"OS error code 53: Invalid request descriptor"
"OS error code 54: Exchange full"
"OS error code 55: No anode"
"OS error code 56: Invalid request code"
"OS error code 57: Invalid slot"
"OS error code 59: Bad font file format"
"OS error code 60: Device not a stream"
"OS error code 61: No data available"
"OS error code 62: Timer expired"
"OS error code 63: Out of streams resources"
"OS error code 64: Machine is not on the network"
"OS error code 65: Package not installed"
"OS error code 66: Object is remote"
"OS error code 67: Link has been severed"
"OS error code 68: Advertise error"
"OS error code 69: Srmount error"
"OS error code 70: Communication error on send"
"OS error code 71: Protocol error"
"OS error code 72: Multihop attempted"
"OS error code 73: RFS specific error"
"OS error code 74: Bad message"
"OS error code 75: Value too large for defined data type"
"OS error code 76: Name not unique on network"
"OS error code 77: File descriptor in bad state"
"OS error code 78: Remote address changed"
"OS error code 79: Can not access a needed shared library"
"OS error code 80: Accessing a corrupted shared library"
"OS error code 81: .lib section in a.out corrupted"
"OS error code 82: Attempting to link in too many shared libraries"
"OS error code 83: Cannot exec a shared library directly"
"OS error code 84: Invalid or incomplete multibyte or wide character"
"OS error code 85: Interrupted system call should be restarted"
"OS error code 86: Streams pipe error"
"OS error code 87: Too many users"
"OS error code 88: Socket operation on non-socket"
"OS error code 89: Destination address required"
"OS error code 90: Message too long"
"OS error code 91: Protocol wrong type for socket"
"OS error code 92: Protocol not available"
"OS error code 93: Protocol not supported"
"OS error code 94: Socket type not supported"
"OS error code 95: Operation not supported"
"OS error code 96: Protocol family not supported"
"OS error code 97: Address family not supported by protocol"
"OS error code 98: Address already in use"
"OS error code 99: Cannot assign requested address"
"OS error code 100: Network is down"
"OS error code 101: Network is unreachable"
"OS error code 102: Network dropped connection on reset"
"OS error code 103: Software caused connection abort"
"OS error code 104: Connection reset by peer"
"OS error code 105: No buffer space available"
"OS error code 106: Transport endpoint is already connected"
"OS error code 107: Transport endpoint is not connected"
"OS error code 108: Cannot send after transport endpoint shutdown"
"OS error code 109: Too many references: cannot splice"
"OS error code 110: Connection timed out"
"OS error code 111: Connection refused"
"OS error code 112: Host is down"
"OS error code 113: No route to host"
"OS error code 114: Operation already in progress"
"OS error code 115: Operation now in progress"
"OS error code 116: Stale NFS file handle"
"OS error code 117: Structure needs cleaning"
"OS error code 118: Not a XENIX named type file"
"OS error code 119: No XENIX semaphores available"
"OS error code 120: Is a named type file"
"OS error code 121: Remote I/O error"
"OS error code 122: Disk quota exceeded"
"OS error code 123: No medium found"
"OS error code 124: Wrong medium type"
"OS error code 125: Operation canceled"
"OS error code 126: Required key not available"
"OS error code 127: Key has expired"
"OS error code 128: Key has been revoked"
"OS error code 129: Key was rejected by service"
"OS error code 130: Owner died"
"OS error code 131: State not recoverable"
python运算符
空串、None、0还有一些表示空性值的会转换为False，其余全部都是True.
逻辑与都为True时为True,如果有一个False 就是False
a=True and True 得True
b=True and False 得False
c=False and False 得True
逻辑或
a=True or True 得True
b=True or False 得True
c=False or False 得False
与运算规则是找False,如果第一个值是False 那么不瞅第二个值，直接输出第一个值，如果第一个值是True,那么直接输出第二个值。
a=1 and 2 得2
a=0 and 2 得0
a=None and 0 得None
a=25 and 254 得254
a=0 and 34 得0
或运算规则是找True,如果第一个值是True 那么不瞧第二个值，直接输出第一个值，如果第一个是值是False,那么直接输出第二个值
a=1 or 2 得 1
a= False or 1 得 1
a= 0 or None 得None
根椐查表判断运算符优先级，and 优先级高于or
r=34 and 89 or 63 得 89
r=0 and 89 or 63 得63
r=None and 89 or 63 得63
r=78 and False or 63 得63
语法：语句1 if 条件表达式 else 语句2
a=25
b=14
c=a if ab else b
print(c)
25
name='521 {}嫁给我好吗？'.format('王奕菲')
name1='521 %s嫁给我好吗？'%'王奕菲'
a='王奕菲'
name2=f'521 {a}嫁给我好吗？'
b='521'
c='嫁给我好吗？'
name3=b+' '+a+c
print(name)
print(name1)
print(name2)
print(name3)
a=54
b=23
c=87
d = a if ab else b
e = c if cd else d
print(e)
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