How to Use String.Replace() in Python 3.X

How to use string.replace() in python 3.x

As in 2.x, use str.replace().

Example:

>>> 'Hello world'.replace('world', 'Guido')
'Hello Guido'

Replace \\ with \ in Python 3.x

In case you're in a python shell and just evaluated the dir, it's sufficient to print(dir).

Otherwise you'd want to do print(dir.replace('\\\\', '\\')), which seems to be a very strange output from os.getcwd()

Keep in mind that '\' is an escape character and thus needs to be escaped itself.

Here's an example from Python shell:

>>> a = 'c:\\foo\\bar'
>>> a
'c:\\foo\\bar'
>>> print(a)
c:\foo\bar

How to replace multiple substrings of a string?

Here is a short example that should do the trick with regular expressions:

import re

rep = {"condition1": "", "condition2": "text"} # define desired replacements here

# use these three lines to do the replacement
rep = dict((re.escape(k), v) for k, v in rep.iteritems()) 
#Python 3 renamed dict.iteritems to dict.items so use rep.items() for latest versions
pattern = re.compile("|".join(rep.keys()))
text = pattern.sub(lambda m: rep[re.escape(m.group(0))], text)

For example:

>>> pattern.sub(lambda m: rep[re.escape(m.group(0))], "(condition1) and --condition2--")
'() and --text--'

Python3.x: quickest way to replace characters in very large string

A more memory efficient method, that will not generate so many temporary strings along the way, would be to use str.translate.

>>> string1 = "XYZYXZZXYZZXYZYXYXZYXZYXZYZYZXY"
>>> string1.translate({ord("X"): "A", ord("Y"): "B", ord("Z"): "C"})
'ABCBACCABCCABCBABACBACBACBCBCAB'

This will allocate just one (extra large in your case) string.

how does replace function work in python 3.x

The documentation for str.replace says:

Return a copy of the string with all occurrences of substring old replaced by new.

From here:

There is also no mutable string type, but str.join() or io.StringIO can be used to efficiently construct strings from multiple fragments.

However, it looks like, at least in the standard implementation, replace is implemented in native code.

String replacement on a whole text file in Python 3.x?

fileinput is the module from the Python standard library that supports "what looks like in-place updating of text files" as well as various other related tasks.

for line in fileinput.input(['thefile.txt'], inplace=True):
    print(line.replace('old stuff', 'shiny new stuff'), end='')

This code is all you need for the specific task you mentioned -- it deals with all of the issues (writing to a different file, removing the old one when done and replacing it with the new one). You can also add a further parameter such as backup='.bk' to automatically preserve the old file as (in this case) thefile.txt.bk, as well as process multiple files, take the filenames to process from the commandline, etc, etc -- do read the docs, they're quite good (and so is the module I'm suggesting!-).

Python string.replace, only replace under certain circumstances

text = '1 x Sandwich, "2 x Coffee, with cream", 1 x Apple pie'

def comma_changer(text):
  text = list(text)
  quote_counter = 0
  for i,char in enumerate(text):
    if char == '"':
      quote_counter+=1
    elif char == ",":
      if quote_counter%2 == 1:
        text[i] = ":"
  return("".join(text))

comma_changer(text) #'1 x Sandwich, "2 x Coffee: with cream", 1 x Apple pie'

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