Question

I have strings stored in various objects, for example a, b, c[2:7] (ie 5 different strings c[2],…c[6]), d.s, e.s[:] (ie all strings stored in the e.s list). I need to loop over all these strings on various places in the code and modify these variables in the loops. (Assigning the modified strings to new variables would not work as there are other functions expecting the modified strings in the original variables.) How can I do that efficiently in Python?

If I could create a list of references to the strings then I could just always iterate over it:

str_ref_lst=[reference(a), reference(b), reference(d.s)]
str_ref_lst.extend[reference(c[2:7])]
str_ref_lst.extend[reference(e.s[:])]
...
for ref_strng in str_ref_lst:
  do_string_modifications1(dereference(ref_strng))
...
function_using_the_modified_variables1()
...
for ref_strng in str_ref_lst:
  do_string_modifications2(dereference(ref_strng))
...
function_using_the_modified_variables2()
etc

However, I am not aware of reference()/dereference() analogues in Python (and I guess it would not be the right way in Python anyway)

Answer 1

The recommended answer should be “use a dictionary”, but if you have to do it that way:

class cls(object):
    pass

a = "<a>"
b = "<b>"
c = ["", "", "<c>"]
d = cls()
d.s = "<d.s>"
e = cls()
e.s = "<e.s>"

var_list = "a b c[2:7] d.s e.s".split()

for var in var_list:
    exec "%s+='Mod'" % var             # Modify the variables

print a,b,c,d.s,e.s
# >>> <a>Mod <b>Mod ['', '', '<c>', 'M', 'o', 'd'] <d.s>Mod <e.s>Mod

IIRC this way doesn’t work in Python 3.0 because of internal optimizations

With slices, it should be somewhat like

import re

class cls(object):
    pass

def fn(var):      # function to apply to the strings
    return "[%s mod]" % var

a = "<a>"
b = "<b>"
c = "01234567890"
d = cls()
d.s = "<d.s>"
e = cls()
e.s = "<e.s>"

var_list = "a b c[2:7] d.s e.s".split()

for var in var_list:
    # Modify the variables
    try:
        # exec "%s='Hi there'" % var             # Modify the variables
        exec "%(var)s=fn(%(var)s)" % { "var": var }
    except TypeError:
        # Assume Slice...
        slice_ = re.findall("\[(\d+)\:(\d+)\]", var)[0]   # Get slice
        var_with_slice = var
        var = var.split("[")[0]                 # Remove slice
        # Look ma, I can code in Perl too! :)
        line = \
             "%(var)s=%(var)s[:%(slice1)s]+fn(%(var_with_slice)s)+%(var)s[%(slice2)s+1:]" % \
             { "var": var, "var_with_slice": var_with_slice, "slice1": slice_[0], "slice2": slice_[1] }
        # somewhat like c = c[:2] + fn(c[2:7]) + c[7+1:]
        exec line

print a,b,c,d.s,e.s
# >>> [<a> mod] [<b> mod] 01[23456 mod]890 [<d.s> mod] [<e.s> mod]