Осуществление
Приведенные ниже функции позволят вставить одну строку в другую строку:
def str_insert(from_me, into_me, at):
"""
Inserts the string <from_me> into <into_me>
Input <at> must be an integer index of <into_me> or a substring of <into_me>
Inserts <from_me> AFTER <at>, not before <at>
Inputs <from_me> and <into_me> must have working __str__ methods defined.
This is satisfied if they already are strings.
If not already strings, <from_me>, <into_me> are converted into strings.
If you try to insert an empty string, that's fine, and the result
is no different from the original.
In order to insert 'from_me' after nothing (insert at the beginning of the string) use:
at = '' or at = 0
"""
try:
return str_insert_or_raise(from_me, into_me, at)
except ValueError as err:
serr = str(err)
if (str_insert_or_raise.__name__ in serr) and 'not found' in serr and '<at>' in serr:
# if can't find where to insert stuff, don't bother to insert it
# use str_insert_or_raise if you want an exception instead
return into_me
else:
raise err
##############################################################
def str_insert_or_raise(from_me, into_me, at):
"""
Inserts the string <from_me> into <into_me>
Inserts <from_me> AFTER <at>, not before <at>
Input <at> must be an integer index of <into_me> or a substring of <into_me>
If <at> is the string '15', that substring will be searched for,
'15' will not be interpreted as an index/subscript.
Inputs <from_me> and <into_me> must have working __str__ methods defined.
If not already strings, <from_me>, <into_me> are converted into strings.
If you try to insert something, but we cannot find the position where
you said to insert it, then an exception is thrown guaranteed to at least
contain the following three substrings:
str_insert_or_raise.__name__
'not found'
'<at>'
"""
try:
if isinstance(at, int):
return str_insert_by_int(from_me, into_me, at)
# Below, the calls to str() work fine if <at> and <from_me> are already strings
# it makes them strings if they are not already
return str_insert_by_str(str(from_me), str(into_me), str(at))
except ValueError as err:
serr = str(err)
if 'empty string' in serr:
return into_me # We allow insertion of the empty string
elif ("<at>" in serr) and 'not found' in serr:
msg_start = "In " + str_insert_or_raise.__name__ + ": "
msg = [msg_start, "\ninput ", "<at> string", " not found in ", "<into_me>",
"\ninput <", str(at) , "> not found in <", str(into_me), ">"]
msg = ''.join(msg)
raise ValueError(msg) from None
else:
raise err
#############################################################
def str_insert_by_str(from_me, into_me, at):
"""
Inserts the string <from_me> into <into_me>
puts 'from_me' AFTER 'at', not before 'at'
For example,
str_insert_or_raise(at = '2', from_me = '0', into_me = '123')
puts the zero after the 2, not before the 2
The call returns '1203' not '1023'
Throws exceptions if input arguments are not strings.
Also, if <from_me> is empty or <at> is not a substring of <into_me> then
an exception is raised.
For fewer exceptions, use <str_insert_or_raise> instead.
"""
try:
s = into_me.replace(at, at + from_me, 1)
except TypeError as terr: # inputs to replace are not strings
msg_list = ['Inputs to function ', str_insert_by_str.__name__, '() must be strings']
raise TypeError(''.join(msg_list)) from None
# At the end of call to replace(), the '1' indicates we will replace
# the leftmost occurrence of <at>, instead of every occurrence of <at>
if (s == into_me): # <at> string not found and/or <from_me> is the empty string
msg_start = "In " + str_insert_by_str.__name__ + ": "
if from_me == '':
msg = ''.join([msg_start, "attempted to insert an empty string"])
raise ValueError(msg) from None
raise ValueError(msg_start, "Input <at> string not found in <into_me>.",
"\nUnable to determine where you want the substring inserted.") from None
return s
##################################################
def str_insert_by_int(from_me, into_me, at):
"""
* Inserts the string <from_me> into <into_me> at integer index <at>
* throws exceptions if input arguments are not strings.
* Also, throws an exception if you try to insert the empty string
* If <at> is less than zero, <from_me> gets placed at the
beginning of <into_me>
* If <at> is greater than the largest index of <into_me>,
<from_me> gets placed after the end of <into_me>
For fewer exceptions, use <str_insert_or_raise> instead.
"""
at = into_me[:(at if at > 0 else 0)]
return str_insert_by_str(from_me, into_me, at)
Использование
Приведенный ниже код демонстрирует, как вызвать функцию str_insert, заданную ранее
def foo(*args):
return args
F = 'F. '
s = 'Using the string \'John \' to specify where to make the insertion'
result = str_insert(from_me = F, into_me ='John Kennedy', at ='John ')
print(foo('\n\n', s, '\n', result))
s = 'Using an int returned by find(\'Ken\') to specify where to make the insertion'
index = 'John Kennedy'.find('Ken') # returns the position of the first letter of 'Ken', not the last letter
result = str_insert(from_me = F, into_me ='John Kennedy', at = index)
print(foo('\n\n', s, '\n', result))
s = 'Using an int (5) to specify where to make the insertion.'
result = str_insert(from_me = F, into_me ='John Kennedy', at = 5)
print(foo('\n\n', s, '\n', result))
s = "Looking for an 'at' string which does not exist"
result = str_insert(from_me = F, into_me ='John Kennedy', at ='x')
print(foo('\n\n', s, '\n', result))
s = ''.join(["Looking for the empty string.",
"\nFind one immediately at the beginning of the string"])
result = str_insert(from_me = F, into_me ='John Kennedy', at = '')
print(foo('\n\n', s, '\n', result))
s = "Insert an empty string at index 3. No visible change"
result = str_insert(from_me = '', into_me = 'John Kennedy', at = 3)
print(foo('\n\n', s, '\n', result))
for index in [-5, -1, 0, 1, 997, 999]:
s = "index " + str(index)
result = str_insert(from_me = F, into_me = 'John Kennedy', at = index)
print(foo('\n\n', s, '\n', result))
Предупреждение об отсутствии возможности изменить на месте
Ни одна из вышеперечисленных функций не изменит строку «на месте». Каждая из функций возвращает измененную копию строки, но исходная строка остается без изменений.
Например,
s = ''.join(["Below is what we get when we forget ",
"to overwrite the string with the value",
" returned by str_insert_or_raise:"])
examp_str = 'John Kennedy'
str_insert('John ', F, examp_str)
print(foo('\n\n', s, '\n', examp_str))
# examp_str is still 'John Kennedy' without the F