Develop Reference

How to avoid .replace replacing a word that was already replaced

Given a string, I have to reverse every word, but keeping them in their places.
I tried:
def backward_string_by_word(text):
for word in text.split():
text = text.replace(word, word[::-1])
return text
But if I have the string Ciao oaiC, when it try to reverse the second word, it's identical to the first after beeing already reversed, so it replaces it again. How can I avoid this?

You can use join in one line plus generator expression:
text = "test abc 123"
text_reversed_words = " ".join(word[::-1] for word in text.split())

s.replace(x, y) is not the correct method to use here:
It does two things:
find x in s
replace it with y
But you do not really find anything here, since you already have the word you want to replace. The problem with that is that it starts searching for x from the beginning at the string each time, not at the position you are currently at, so it finds the word you have already replaced, not the one you want to replace next.
The simplest solution is to collect the reversed words in a list, and then build a new string out of this list by concatenating all reversed words. You can concatenate a list of strings and separate them with spaces by using ' '.join().
def backward_string_by_word(text):
reversed_words = []
for word in text.split():
reversed_words.append(word[::-1])
return ' '.join(reversed_words)
If you have understood this, you can also write it more concisely by skipping the intermediate list with a generator expression:
def backward_string_by_word(text):
return ' '.join(word[::-1] for word in text.split())

Splitting a string converts it to a list. You can just reassign each value of that list to the reverse of that item. See below:
text = "The cat tac in the hat"
def backwards(text):
split_word = text.split()
for i in range(len(split_word)):
split_word[i] = split_word[i][::-1]
return ' '.join(split_word)
print(backwards(text))

a mistake I keep having with for loops and return statements

I have been noticing a problem I am having whenever I try to make a function that takes changes a string or a list then returns it.
I will give you an example of this happening with a code I just wrote:
def remove_exclamation(string):
string.split(' ')
for i in string:
i.split()
for char in i:
if char == '!':
del char
''.join(i)
' '.join(string)
return string
For instance, I create this code to take a string as its parameter, remove any exclamation in it, the return it changed. The input and output should look like this:
>>>remove_exclamation('This is an example!')
'This is an example'
But instead I get this:
>>>remove_exclamation('This is an example!')
'This is an example!'
The function is not removing the exclamation in the output, and is not doing what I intended for it to day.
How can I keep avoiding this when I make for loops, nested for loops etc?

You write your code and formulate your question as if it was possible to modify strings in Python. It is not possible.
Strings are immutable. All functions which operate on strings return new strings. They do not modify existing strings.
This returns a list of strings, but you are not using the result:
string.split(' ')
This also:
i.split()
This deletes the variable named char. It does not affect the char itself:
del char
This creates a new string which you do not use:
''.join(i)
This also:
' '.join(string)
All in all, almost every line of the code is wrong.
You probably wanted to do this:
def remove_exclamation(string):
words = string.split(' ')
rtn_words = []
for word in words:
word_without_exclamation = ''.join(ch for ch in word if ch != '!')
rtn_words.append(word_without_exclamation)
return ' '.join(rtn_words)
But in the end, this does the same thing:
def remove_exclamation(string):
return string.replace('!', '')

Without clearly knowing the intentions of your function and what you are attempting to do. I have an alternative to the answer that zvone gave.
This option is to remove any characters that you have not defined in an allowed characters list:
characters = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ "
test_string = "This is an example!"
test_string = ''.join(list(filter(lambda x: x in characters, test_string)))
print(test_string)
This outputs:
This is an example
Note, this is the Python 3 version.
Python 2, you do not need the ''.join(list())
Doing it this way would allow you to define any character that you do not want present in your string, and it will remove them.
You can even do the reverse:
ignore_characters= "!"
test_string = "This is an example!"
test_string = ''.join(list(filter(lambda x: x not in ignore_characters, test_string)))
print(test_string)

Strings are immutable in Python. And you cannot change them. You can however, re-assign there values.
That is where your problem lies. You never reassign the value of your strings, when you call .split() on them.
But there are also others errors in your program such as:
Your indention
The fact that your just returning the string thats passed into the function
Your use of the del statement
etc.
Instead, create a new string by iterating through the old one and filtering out the character(s) you do not want, via list comprehension and ''.join().
def remove_exclamation(string):
return ''.join([char for char in string if char != '!'])
But as #Moses has already said in the comments, why not just use str.replace()?:
string = string.replace('!', '')

def remove_exclamation(string):
#you think you are splitting string into tokens
#but you are not assigning the split anywhere...
string.split(' ')
#and here you are cycling through individual _chars_ in string which was not affected by the split above ;-)
for i in string:
#and now you are splitting a 1-char string and again not assigning it.
i.split()
And string is still your input param, which I assume is of type str. And immutable.
On top of which, if you were import/using the string module, you would be shadowing it
A big part of your confusion is knowing when the methods mutate the objects and when they return a new object. In the case of strings, they never mutate, you need to assign the results to a new variable.
On a list however, and the join() somewhere makes me think you want to use a list, then methods generally change the object in place.
Anyway, on to your question:
def remove_exclamation(inputstring, to_remove="!"):
return "".join([c for c in inputstring if c != to_remove])
print (remove_exclamation('This is an example!'))
output:
This is an example

How to encode a python list

I'm having a hard time trying to encode a python list, I already did it with a text file in order to count specific words inside it, using re module.
This is the code:
# encoding text file
with codecs.open('projectsinline.txt', 'r', encoding="utf-8") as f:
for line in f:
# Using re module to extract specific words
unicode_pattern = re.compile(r'\b\w{4,20}\b', re.UNICODE)
result = unicode_pattern.findall(line)
word_counts = Counter(result) # It creates a dictionary key and wordCount
Allwords = []
for clave in word_counts:
if word_counts[clave] >= 10: # We look for the most repeated words
word = clave
Allwords.append(word)
print Allwords
Part of the output looks like this:
[...u'recursos', u'Partidos', u'Constituci\xf3n', u'veh\xedculos', u'investigaci\xf3n', u'Pol\xedticos']
If I print variable word the output looks as it should be. However, when I use append, all the words breaks again, as the example before.
I use this example:
[x.encode("utf-8") for x in Allwords]
The output looks exactly the same as before.
I also use this example:
Allwords.append(str(word.encode("utf-8")))
The output change, but the words don't look as they should be:
[...'recursos', 'Partidos', 'Constituci\xc3\xb3n', 'veh\xc3\xadculos', 'investigaci\xc3\xb3n', 'Pol\xc3\xadticos']
Some of the answers have given this example:
print('[' + ', '.join(Allwords) + ']')
The output looks like this:
[...recursos, Partidos, Constitución, vehículos, investigación, Políticos]
To be honest I do not want to print the list, just encode it, so that all items (words) are recognized.
I'm looking for something like this:
[...'recursos', 'Partidos', 'Constitución', 'vehículos', 'investigación', 'Políticos']
Any suggestions to solve the problem are appreciated
Thanks,

you might what to try
print('[' + ', '.join(Allwords) + ']')

Your Unicode string list is correct. When you print lists the items in the list display as their repr() function. When you print the items themselves, the items display as their str() function. It is only a display option, similar to printing integers as decimal or hexadecimal.
So print the individual words if you want to see them correctly, but for comparisons the content is correct.
It's worth noting that Python 3 changes the behavior of repr() and now will display non-ASCII characters without escape codes if the terminal supports them directly and the ascii() function reproduces the Python 2 repr() behavior.

How do I output the acronym on one line

I am following the hands-on python tutorials from Loyola university and for one exercise I am supposed to get a phrase from the user, capatalize the first letter of each word and print the acronym on one line.
I have figured out how to print the acronym but I can't figure out how to print all the letters on one line.
letters = []
line = input('?:')
letters.append(line)
for l in line.split():
print(l[0].upper())

Pass end='' to your print function to suppress the newline character, viz:
for l in line.split():
print(l[0].upper(), end='')
print()

Your question would be better if you shared the code you are using so far, I'm just guessing that you have saved the capital letters into a list.
You want the string method .join(), which takes a string separator before the . and then joins a list of items with that string separator between them. For an acronym you'd want empty quotes
e.g.
l = ['A','A','R','P']
acronym = ''.join(l)
print(acronym)

You could make a string variable at the beginning string = "".
Then instead of doing print(l[0].upper()) just append to the string string += #yourstuff
Lastly, print(string)

separate line in words by slash and use /W but avoid :

i am trying to parse a txt file with a lot of lines like this:
470115572 cms_trk_dcs_05:CAEN/CMS_TRACKER_SY1527_7/branchController00/easyCrate3/easyBoard16/channel003
i am making a dictionary where the key is the first number on the line, and the values are (for each key) the words separated by the slash "/", every one of this words is saved into a list, for example list1 gets all cms_trk_dcs_05:CAEN, list2 would be all CMS_TRACKER_SY1527_7, etc
but when i use pattern = re.split('\W',line) to split the line, it takes into account
the ":" character, i mean when i try to print cms_trk_dcs_05:CAEN it only returns cms_trk_dcs_05, how can i save in the list all the word cms_trk_dcs_05:CAEN, and save in my list all the words separated by slash
I am new at python, so i apologize if this is for dummys
anyway thank you in advance

Use split() to match first the space after the number, and then the '/':
>>> stringin = "470115572 cms_trk_dcs_05:CAEN/CMS_TRACKER_SY1527_7/branchController00/easyCrate3/easyBoard16/channel003"
>>> splitstring = stringin.split(' ')
>>> num = splitstring[0]
>>> stringlist = splitstring[1].split('/')
>>> num
'470115572'
>>> stringlist
['cms_trk_dcs_05:CAEN', 'CMS_TRACKER_SY1527_7', 'branchController00', 'easyCrate3', 'easyBoard16', 'channel003']
>>>
Or as a (less obvious) one-liner:
>>> [x.split('/') for x in stringin.split(' ')]
[['470115572'], ['cms_trk_dcs_05:CAEN', 'CMS_TRACKER_SY1527_7', 'branchController00', 'easyCrate3', 'easyBoard16', 'channel003']]
Note, though, that the second approach creates the first element as a list.

As in Trimax's comment: : (colon) is a nonword character, so to split line correctly you need to include it in pattern. Or use SiHa's answer.
About pattern, \W equals to [^a-zA-Z0-9_] (https://docs.python.org/2/library/re.html#regular-expression-syntax), so you can just add colon to it: [^a-zA-Z0-9_:]
As for second part, just use first element of result list as dict key and assign remained list to it in form of slice.
Something like this:
result_dict = {}
for line in file_lines:
line_splitted = re.split('[^a-zA-Z0-9_:]+', line)
result_dict[line_splitted[0]] = line_splitted[1:]
Note though, if your text contains lines with same numbers, you'll lose data, as when assigning new value (list of words in this case) to existing key, it will overwrite previous value.