提出问题

在写 re.sub 或者 re.subn 的时候, 常常会不太确定引用参数的顺序, 需要中断的时间查看提示或者help文档. 比如 trade war 修改为 trade negotiation

In [17]: re.sub("war", "negotiation", "trade war")
Out[17]: 'trade negotiation'
sub(pattern, repl, string, count=0, flags=0)
def sub(pattern, repl, string, count=0, flags=0):
    return _compile(pattern, flags).sub(repl, string, count)#首先处理regex-pattern

分析问题

与 string.methods 比较. pattern 是与source(input_string)的匹配的内容, repl是修改后的内容(destination), 这里顺序与=str.replace=是一致.

replace(self, old, new, count=-1, /)

old 来自source, new是输出到destination结果中.

In [16]: "trade war".replace("war", "negotiation")
Out[16]: 'trade negotiation'

sed 也遵循同样的模式.

s/regexp/replacement/
       Attempt  to  match  regexp  against  the  pattern  space.   If successful, replace that portion matched with
       replacement.  The replacement may contain the special character & to refer to that portion  of  the  pattern
       space  which  matched,  and  the  special  escapes \1 through \9 to refer to the corresponding matching sub-
       expressions in the regexp.

regex-pattern匹配 source 数据中的内容, replacement则是替换后输出到destination结果中.

$ echo 'trade war' | sed "s/war/negotiation/g"
trade negotiation

其他的Text Processing

$ echo "trade-war" | tr "-" "\n"
trade
war
   tr [OPTION]... SET1 [SET2]

SET1 is from the source, SET2 is the result of the destination after been processed.

总结这种模式和思维惯例:

function source destination

Text Processing如此,

File Handling的utilities遵循同样的模式.

mv [OPTION]... SOURCE... DIRECTORY
cp [OPTION]... [-T] SOURCE DEST
ln [OPTION]... Source... DIRECTORY
rsync [OPTION...] SRC... [DEST]
scp  SRC... DEST
dd if=/dev/{{source_drive}} of=/dev/{{dest_drive}}

例外的情况是tar. =tar=是将目标放在前面.

再回头看 re.sub

re.sub(pattern, repl, string)
#扩展后
re.sub(pattern_from_source, replacement_to_result, source_data)

三个参数中 pattern_from_source, replacement_to_result , source_data 的最后一个是=source_data, 将source放置在最后. =grep 与 =sed=都遵循同样的模式

sed 's/{{regex}}/{{replace}}/' {{filename}}
grep [OPTIONS] -e PATTERN ... [FILE...] #grep regex source

例外的情况是=find=

find [-H] [-L] [-P] [-D debugopts] [-Olevel] [starting-point...] [expression]
find [Option] source pattern

总结:

Data Stream Processing和File Handling遵循 subroutine src dst 模式.两个例外的情况是=tar and find=

这个问题之所以值得探讨,是因为涉及底层的方法论和工作模式.