I'm trying to remove specific characters from a string using Python. This is the code I'm using right now. Unfortunately it appears to do nothing to the string.
for char in line:
if char in " ?.!/;:":
line.replace(char,'')
How do I do this properly?
Strings in Python are immutable (can't be changed). Because of this, the effect of line.replace(...) is just to create a new string, rather than changing the old one. You need to rebind (assign) it to line in order to have that variable take the new value, with those characters removed.
Also, the way you are doing it is going to be kind of slow, relatively. It's also likely to be a bit confusing to experienced pythonators, who will see a doubly-nested structure and think for a moment that something more complicated is going on.
Starting in Python 2.6 and newer Python 2.x versions *, you can instead use str.translate, (see Python 3 answer below):
line = line.translate(None, '!##$')
or regular expression replacement with re.sub
import re
line = re.sub('[!##$]', '', line)
The characters enclosed in brackets constitute a character class. Any characters in line which are in that class are replaced with the second parameter to sub: an empty string.
Python 3 answer
In Python 3, strings are Unicode. You'll have to translate a little differently. kevpie mentions this in a comment on one of the answers, and it's noted in the documentation for str.translate.
When calling the translate method of a Unicode string, you cannot pass the second parameter that we used above. You also can't pass None as the first parameter. Instead, you pass a translation table (usually a dictionary) as the only parameter. This table maps the ordinal values of characters (i.e. the result of calling ord on them) to the ordinal values of the characters which should replace them, or—usefully to us—None to indicate that they should be deleted.
So to do the above dance with a Unicode string you would call something like
translation_table = dict.fromkeys(map(ord, '!##$'), None)
unicode_line = unicode_line.translate(translation_table)
Here dict.fromkeys and map are used to succinctly generate a dictionary containing
{ord('!'): None, ord('#'): None, ...}
Even simpler, as another answer puts it, create the translation table in place:
unicode_line = unicode_line.translate({ord(c): None for c in '!##$'})
Or, as brought up by Joseph Lee, create the same translation table with str.maketrans:
unicode_line = unicode_line.translate(str.maketrans('', '', '!##$'))
* for compatibility with earlier Pythons, you can create a "null" translation table to pass in place of None:
import string
line = line.translate(string.maketrans('', ''), '!##$')
Here string.maketrans is used to create a translation table, which is just a string containing the characters with ordinal values 0 to 255.
Am I missing the point here, or is it just the following:
string = "ab1cd1ef"
string = string.replace("1", "")
print(string)
# result: "abcdef"
Put it in a loop:
a = "a!b#c#d$"
b = "!##$"
for char in b:
a = a.replace(char, "")
print(a)
# result: "abcd"
>>> line = "abc##!?efg12;:?"
>>> ''.join( c for c in line if c not in '?:!/;' )
'abc##efg12'
With re.sub regular expression
Since Python 3.5, substitution using regular expressions re.sub became available:
import re
re.sub('\ |\?|\.|\!|\/|\;|\:', '', line)
Example
import re
line = 'Q: Do I write ;/.??? No!!!'
re.sub('\ |\?|\.|\!|\/|\;|\:', '', line)
'QDoIwriteNo'
Explanation
In regular expressions (regex), | is a logical OR and \ escapes spaces and special characters that might be actual regex commands. Whereas sub stands for substitution, in this case with the empty string ''.
The asker almost had it. Like most things in Python, the answer is simpler than you think.
>>> line = "H E?.LL!/;O:: "
>>> for char in ' ?.!/;:':
... line = line.replace(char,'')
...
>>> print line
HELLO
You don't have to do the nested if/for loop thing, but you DO need to check each character individually.
For the inverse requirement of only allowing certain characters in a string, you can use regular expressions with a set complement operator [^ABCabc]. For example, to remove everything except ascii letters, digits, and the hyphen:
>>> import string
>>> import re
>>>
>>> phrase = ' There were "nine" (9) chick-peas in my pocket!!! '
>>> allow = string.letters + string.digits + '-'
>>> re.sub('[^%s]' % allow, '', phrase)
'Therewerenine9chick-peasinmypocket'
From the python regular expression documentation:
Characters that are not within a range can be matched by complementing
the set. If the first character of the set is '^', all the characters
that are not in the set will be matched. For example, [^5] will match
any character except '5', and [^^] will match any character except
'^'. ^ has no special meaning if it’s not the first character in the
set.
line = line.translate(None, " ?.!/;:")
>>> s = 'a1b2c3'
>>> ''.join(c for c in s if c not in '123')
'abc'
Strings are immutable in Python. The replace method returns a new string after the replacement. Try:
for char in line:
if char in " ?.!/;:":
line = line.replace(char,'')
This is identical to your original code, with the addition of an assignment to line inside the loop.
Note that the string replace() method replaces all of the occurrences of the character in the string, so you can do better by using replace() for each character you want to remove, instead of looping over each character in your string.
I was surprised that no one had yet recommended using the builtin filter function.
import operator
import string # only for the example you could use a custom string
s = "1212edjaq"
Say we want to filter out everything that isn't a number. Using the filter builtin method "...is equivalent to the generator expression (item for item in iterable if function(item))" [Python 3 Builtins: Filter]
sList = list(s)
intsList = list(string.digits)
obj = filter(lambda x: operator.contains(intsList, x), sList)))
In Python 3 this returns
>> <filter object # hex>
To get a printed string,
nums = "".join(list(obj))
print(nums)
>> "1212"
I am not sure how filter ranks in terms of efficiency but it is a good thing to know how to use when doing list comprehensions and such.
UPDATE
Logically, since filter works you could also use list comprehension and from what I have read it is supposed to be more efficient because lambdas are the wall street hedge fund managers of the programming function world. Another plus is that it is a one-liner that doesnt require any imports. For example, using the same string 's' defined above,
num = "".join([i for i in s if i.isdigit()])
That's it. The return will be a string of all the characters that are digits in the original string.
If you have a specific list of acceptable/unacceptable characters you need only adjust the 'if' part of the list comprehension.
target_chars = "".join([i for i in s if i in some_list])
or alternatively,
target_chars = "".join([i for i in s if i not in some_list])
Using filter, you'd just need one line
line = filter(lambda char: char not in " ?.!/;:", line)
This treats the string as an iterable and checks every character if the lambda returns True:
>>> help(filter)
Help on built-in function filter in module __builtin__:
filter(...)
filter(function or None, sequence) -> list, tuple, or string
Return those items of sequence for which function(item) is true. If
function is None, return the items that are true. If sequence is a tuple
or string, return the same type, else return a list.
Try this one:
def rm_char(original_str, need2rm):
''' Remove charecters in "need2rm" from "original_str" '''
return original_str.translate(str.maketrans('','',need2rm))
This method works well in Python 3
Here's some possible ways to achieve this task:
def attempt1(string):
return "".join([v for v in string if v not in ("a", "e", "i", "o", "u")])
def attempt2(string):
for v in ("a", "e", "i", "o", "u"):
string = string.replace(v, "")
return string
def attempt3(string):
import re
for v in ("a", "e", "i", "o", "u"):
string = re.sub(v, "", string)
return string
def attempt4(string):
return string.replace("a", "").replace("e", "").replace("i", "").replace("o", "").replace("u", "")
for attempt in [attempt1, attempt2, attempt3, attempt4]:
print(attempt("murcielago"))
PS: Instead using " ?.!/;:" the examples use the vowels... and yeah, "murcielago" is the Spanish word to say bat... funny word as it contains all the vowels :)
PS2: If you're interested on performance you could measure these attempts with a simple code like:
import timeit
K = 1000000
for i in range(1,5):
t = timeit.Timer(
f"attempt{i}('murcielago')",
setup=f"from __main__ import attempt{i}"
).repeat(1, K)
print(f"attempt{i}",min(t))
In my box you'd get:
attempt1 2.2334518376057244
attempt2 1.8806643818474513
attempt3 7.214925774955572
attempt4 1.7271184513757465
So it seems attempt4 is the fastest one for this particular input.
Here's my Python 2/3 compatible version. Since the translate api has changed.
def remove(str_, chars):
"""Removes each char in `chars` from `str_`.
Args:
str_: String to remove characters from
chars: String of to-be removed characters
Returns:
A copy of str_ with `chars` removed
Example:
remove("What?!?: darn;", " ?.!:;") => 'Whatdarn'
"""
try:
# Python2.x
return str_.translate(None, chars)
except TypeError:
# Python 3.x
table = {ord(char): None for char in chars}
return str_.translate(table)
#!/usr/bin/python
import re
strs = "how^ much for{} the maple syrup? $20.99? That's[] ricidulous!!!"
print strs
nstr = re.sub(r'[?|$|.|!|a|b]',r' ',strs)#i have taken special character to remove but any #character can be added here
print nstr
nestr = re.sub(r'[^a-zA-Z0-9 ]',r'',nstr)#for removing special character
print nestr
You can also use a function in order to substitute different kind of regular expression or other pattern with the use of a list. With that, you can mixed regular expression, character class, and really basic text pattern. It's really useful when you need to substitute a lot of elements like HTML ones.
*NB: works with Python 3.x
import re # Regular expression library
def string_cleanup(x, notwanted):
for item in notwanted:
x = re.sub(item, '', x)
return x
line = "<title>My example: <strong>A text %very% $clean!!</strong></title>"
print("Uncleaned: ", line)
# Get rid of html elements
html_elements = ["<title>", "</title>", "<strong>", "</strong>"]
line = string_cleanup(line, html_elements)
print("1st clean: ", line)
# Get rid of special characters
special_chars = ["[!##$]", "%"]
line = string_cleanup(line, special_chars)
print("2nd clean: ", line)
In the function string_cleanup, it takes your string x and your list notwanted as arguments. For each item in that list of elements or pattern, if a substitute is needed it will be done.
The output:
Uncleaned: <title>My example: <strong>A text %very% $clean!!</strong></title>
1st clean: My example: A text %very% $clean!!
2nd clean: My example: A text very clean
My method I'd use probably wouldn't work as efficiently, but it is massively simple. I can remove multiple characters at different positions all at once, using slicing and formatting.
Here's an example:
words = "things"
removed = "%s%s" % (words[:3], words[-1:])
This will result in 'removed' holding the word 'this'.
Formatting can be very helpful for printing variables midway through a print string. It can insert any data type using a % followed by the variable's data type; all data types can use %s, and floats (aka decimals) and integers can use %d.
Slicing can be used for intricate control over strings. When I put words[:3], it allows me to select all the characters in the string from the beginning (the colon is before the number, this will mean 'from the beginning to') to the 4th character (it includes the 4th character). The reason 3 equals till the 4th position is because Python starts at 0. Then, when I put word[-1:], it means the 2nd last character to the end (the colon is behind the number). Putting -1 will make Python count from the last character, rather than the first. Again, Python will start at 0. So, word[-1:] basically means 'from the second last character to the end of the string.
So, by cutting off the characters before the character I want to remove and the characters after and sandwiching them together, I can remove the unwanted character. Think of it like a sausage. In the middle it's dirty, so I want to get rid of it. I simply cut off the two ends I want then put them together without the unwanted part in the middle.
If I want to remove multiple consecutive characters, I simply shift the numbers around in the [] (slicing part). Or if I want to remove multiple characters from different positions, I can simply sandwich together multiple slices at once.
Examples:
words = "control"
removed = "%s%s" % (words[:2], words[-2:])
removed equals 'cool'.
words = "impacts"
removed = "%s%s%s" % (words[1], words[3:5], words[-1])
removed equals 'macs'.
In this case, [3:5] means character at position 3 through character at position 5 (excluding the character at the final position).
Remember, Python starts counting at 0, so you will need to as well.
In Python 3.5
e.g.,
os.rename(file_name, file_name.translate({ord(c): None for c in '0123456789'}))
To remove all the number from the string
How about this:
def text_cleanup(text):
new = ""
for i in text:
if i not in " ?.!/;:":
new += i
return new
Below one.. with out using regular expression concept..
ipstring ="text with symbols!##$^&*( ends here"
opstring=''
for i in ipstring:
if i.isalnum()==1 or i==' ':
opstring+=i
pass
print opstring
Recursive split:
s=string ; chars=chars to remove
def strip(s,chars):
if len(s)==1:
return "" if s in chars else s
return strip(s[0:int(len(s)/2)],chars) + strip(s[int(len(s)/2):len(s)],chars)
example:
print(strip("Hello!","lo")) #He!
You could use the re module's regular expression replacement. Using the ^ expression allows you to pick exactly what you want from your string.
import re
text = "This is absurd!"
text = re.sub("[^a-zA-Z]","",text) # Keeps only Alphabets
print(text)
Output to this would be "Thisisabsurd". Only things specified after the ^ symbol will appear.
# for each file on a directory, rename filename
file_list = os.listdir (r"D:\Dev\Python")
for file_name in file_list:
os.rename(file_name, re.sub(r'\d+','',file_name))
Even the below approach works
line = "a,b,c,d,e"
alpha = list(line)
while ',' in alpha:
alpha.remove(',')
finalString = ''.join(alpha)
print(finalString)
output: abcde
The string method replace does not modify the original string. It leaves the original alone and returns a modified copy.
What you want is something like: line = line.replace(char,'')
def replace_all(line, )for char in line:
if char in " ?.!/;:":
line = line.replace(char,'')
return line
However, creating a new string each and every time that a character is removed is very inefficient. I recommend the following instead:
def replace_all(line, baddies, *):
"""
The following is documentation on how to use the class,
without reference to the implementation details:
For implementation notes, please see comments begining with `#`
in the source file.
[*crickets chirp*]
"""
is_bad = lambda ch, baddies=baddies: return ch in baddies
filter_baddies = lambda ch, *, is_bad=is_bad: "" if is_bad(ch) else ch
mahp = replace_all.map(filter_baddies, line)
return replace_all.join('', join(mahp))
# -------------------------------------------------
# WHY `baddies=baddies`?!?
# `is_bad=is_bad`
# -------------------------------------------------
# Default arguments to a lambda function are evaluated
# at the same time as when a lambda function is
# **defined**.
#
# global variables of a lambda function
# are evaluated when the lambda function is
# **called**
#
# The following prints "as yellow as snow"
#
# fleece_color = "white"
# little_lamb = lambda end: return "as " + fleece_color + end
#
# # sometime later...
#
# fleece_color = "yellow"
# print(little_lamb(" as snow"))
# --------------------------------------------------
replace_all.map = map
replace_all.join = str.join
If you want your string to be just allowed characters by using ASCII codes, you can use this piece of code:
for char in s:
if ord(char) < 96 or ord(char) > 123:
s = s.replace(char, "")
It will remove all the characters beyond a....z even upper cases.
I'm trying to add to a string over a few function calls that you could basically say will "update" the string. So for example, if you had:
'This is a string'
You could change it to:
'This is my string'
Or then:
'This is my string here'
etc..
My data for the string is coming from a nested dictionary, and I made a function that will change it to a string. This function is called 'create_string()'. I won't post it because it is working fine (although if necessary, I'll make an edit. But take my word for it that it's working fine).
Here's the function 'updater()' which takes three arguments: The string, the position you want to change and the string you want to insert.
def updater(c_string, val, position):
data = c_string.split(' ')
data[position] = str(val)
string = ' '.join(data)
return string
x = create_string(....)
new_string = updater(x,'hey', 0)
Which up until this point works fine:
'hey This is a string'
But when you add another function call, it doesn't keep track of the old string:
new_string = updater(x,'hey',0)
new_string = updater(x,'hi',2)
> 'This is hi string'
I know that the reason is likely because of the variable assignment, but i tried just simply calling the functions, and I still had no luck.
How can I get this working?
Thanks for the help!
Note: Please don't waste your time on the create_string() function, it's working fine. It's only the updater() function and maybe even just the function calls that I think are the problem.
**Edit:**Here's what the expected output would look like:
new_string = updater(x,'hey',0)
new_string = updater(x,'hi',2)
> 'hey is hi string'
You need to do this, to keep modifying the string:
new_string = updater(x, 'hey', 0)
new_string = updater(new_string, 'hi', 2)
x is the same after the first call, the new modified string is new_string from that point on.
You store the result of updater to new_string, but don't pass that new_string to the next updater call.
I am learning Python and am working on this exercise:
Create a function that will return another string similar to the input string, but with its case inverted. For example, input of "Mr. Ed" will result in "mR. eD" as the output string.
My code is:
name = 'Mr.Ed'
name_list = []
for i in name:
if i.isupper():
name_list.append(i.lower())
elif i.islower():
name_list.append(i.upper())
else:
name_list.append(i)
print(''.join(name_list))
Is there a simpler or more direct way to solve it?
You can do that with name.swapcase(). Look up the string methods (or see the older docs for legacy Python 2).
Your solution is perfectly fine.
You don't need three branches though, because str.upper() will return str when upper is not applicable anyway.
With generator expressions, this can be shortened to:
>>> name = 'Mr.Ed'
>>> ''.join(c.lower() if c.isupper() else c.upper() for c in name)
'mR.eD'
Simply use the swapcase() method :
name = "Mr.Ed"
name = name.swapcase()
Output : mR.eD
-> This is just a two line code.
Explanation :
The method swapcase() returns a copy of the string in which all the case-based characters have had their case swapped.
Happy Coding!
In python, an inbuilt function swapcase() is present which automatically converts the case of each and every letter. Even after entering the mixture of lowercase and uppercase letters it will handle it properly and return the answer as expected.
Here is my code:
str1=input("enter str= ")
res=str1.swapcase()
print(res)
name='Mr.Ed'
print(name.swapcase())
https://github.com/suryashekhawat/pythonExamples/blob/master/string_toggle.py
def toggle(mystr):
arr = []
for char in mystr:
if char.upper() != char:
char=char.upper()
arr.append(char)
else:
char=char.lower()
arr.append(char)
return ''.join(map(str,arr))
user_input = raw_input()
output = toggle(user_input)
print output
#the following program is for toggle case
name=input()
for i in name:
if i.isupper():
print( i.lower(),sep='',end='')
else:
print( i.upper(),sep='',end='')
changeCase lambda method reverses the case of string if the character is alpha then perform (chr(ord(char)^32) which flips the case of a alphabet. ''.join([]) converts the list to a string.
ord returns an Unicode code representing a character.
chr converts an integer to a character.
main.py
#!/usr/bin/env python3.10
changeCase = lambda x: ''.join([(chr(ord(v)^32) if v.isalpha() else v) for v in x])
print(changeCase("Dev Parzival"))
print(changeCase("Hello World!"))
Output:
$ chmod +x main.py
$ ./main.py
dEV pARZIVAL
hELLO wORLD!
I have been trying out (for my own personal use) some peoples' solutions to timed keyboard inputs and the only one that has worked was one by Alex Martelli/martineau here. I used their second block of code (starting with import msvcrt) and it worked great for pretty much everything but comparisons. I replaced the return of None with an empty string if no input is entered in time and I used some test lines as shown below:
import msvcrt
import time
def raw_input_with_timeout(prompt, timeout):
print prompt,
finishat = time.time() + timeout
result = []
while True:
if msvcrt.kbhit():
result.append(msvcrt.getche())
if result[-1] == '\r': # or \n, whatever Win returns;-)
return ''.join(result)
time.sleep(0.1) # just to yield to other processes/threads
else:
if time.time() > finishat:
return ""
textVar = raw_input_with_timeout("Enter here: \n", 5)
print str(textVar) # to make sure the string is being stored
print type(str(textVar)) # to make sure it is of type string and can be compared
print str(str(textVar) == "test")
time.sleep(10) # so I can see the output
After I compile that with pyinstaller, run it, and type test into the window, I get this output:
Enter here:
test
test
<type 'str'>
False
I originally thought the comparison was returning False because the function appends characters to an array and that may have had something to do with it not doing a proper comparison with a string, but after looking further into the way Python works (namely, SilentGhost's response here), I really have no idea why the comparison will not return True. Any response is appreciated. Thank you!
You won't be able to see why the strings are different just by printing. String values can contain bytes that are not (easily) visible on a console when printed.
Use the repr() function to produce a debugging-friendly representation instead. This representation will format the string as a Python string literal, using only printable ASCII characters and escape sequences:
>>> foo = 'test\t\n'
>>> print foo
test
>>> foo == 'test'
False
>>> print repr(foo)
'test\t\n'
In your case, you are including the \r carriage return character in your return value:
if result[-1] == '\r':
return ''.join(result)
That last \r is still there, so you get, at the very least, the value 'test\r', but \r won't show up when printing:
>>> print 'test\r'
test
>>> print repr('test\r')
'test\r'
You could just exclude that last character when joining, by slicing the string:
return ''.join(result[:-1])
or you could use str.strip() to remove all whitespace characters from both the start and end of the string (including that \r character):
return ''.join(result).strip()
Note that there is no point in using str() calls here. You return a str object, so str(textVar) is redundant. Moreover, print will call str() on anything not a string object yet.
If you consider this fragment of code:
result = []
while True:
if msvcrt.kbhit():
result.append(msvcrt.getche())
if result[-1] == '\r': # or \n, whatever Win returns;-)
return ''.join(result)
You can see that when building the input string, the final character that the user enters must be \r, which is an unprintable character corresponding to the carriage return. Therefore, the returned input string looks like:
test\r
I think you need to rework to the code to discard the final unprintable character from the input.
You probably have some unseen bytes after the string. Try to print([c for c in textVar]) and if it shows characters lie '\r' and \n try str(textVar).strip() == "test" or remove those chars manually.
I am using python 2.7. If i have a string assigned to the name variable like below
name = "Test with-name and_underscore"
How can i convert this into a string that can be assigned to the name variable
name = "TestWithNameAndUnderscore"
Is Regular expression the way to go or does python have any built functions to do that....
So what i am looking for is, when a string comes in with underscore or dashes or space or any special char in it, that get converted to the same thing but No underscore/dashes/space/special char and the initial letter of that word would have start with upper case, so like "test name - is this_here" to "TestNameIsThisHere".
And if there is no space or no special chars then dont do anything. So if the string is "Helloworld", to skip it and move on.
Reason why i am doing this is, i am writing something for AWS using python boto and there is a naming restriction on what the resources can be called. It cannot be non alphanumeric
>>> import re
>>> name = "Test with-name and_underscore"
>>> print(''.join(x.capitalize() for x in re.compile(r'[^a-zA-Z0-9]').split(name)))
TestWithNameAndUnderscore
You can strip leading digits off as well, if that is needed. Here is a slightly more robust example that will do that and insure that the resulting string is not empty:
>>> import re
>>> def fix_id(s, split=re.compile('[^a-zA-Z0-9]+|^[0-9]+').split):
... result = ''.join(x.capitalize() for x in split(s))
... if not result:
... raise ValueError('Invalid ID (empty after edits)')
... return result
...
>>> fix_id("Test with-name and_underscore")
'TestWithNameAndUnderscore'
>>> fix_id("123 Test 456 with-name and_underscore 789")
'Test456WithNameAndUnderscore789'
>>> fix_id("Thisshouldbeunmolested")
'Thisshouldbeunmolested'
>>> fix_id('123')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 4, in fix_id
ValueError: Invalid ID (empty after edits)
Note that neither of these will guarantee the uniqueness of your identifiers, e.g. "Mary-Sue" and "mary sue" will map to the same identifier. If you need these to map to different identifiers, you can add a cache dictionary, where you map symbols and add a suffix if necessary.
This can be done without Regex using the isalnum() function in Python.
name = "Test with-name and_underscore"
new_name = ''.join(name for name in string if e.isalnum())
Ofcourse, if you do insist on using regex this is possible too by replacing isalnum() with the appropriate regex function.
I know a stupid way to do it!
name.replace('_',' ').replace('-',' ')
name = name.title().replace(' ','')
A possible smaller re approach would be to use the following:
import re
string = '123 this is a test_of the-sub method 33'
varString = re.sub('_?-? ?', '', string)
it should return
>>> sub('_?-? ?','',string)
'123thisisatestofthesubmethod33'
If you are trying to use it as a variable name though you likely will run into some troubles, such as too long (pep8 compliance) or other foreign characters such as !?$% etc....in which the isalpha method above may help with. I'd be careful us trusting the value of the string to become a variable name and wrap some constraints to avoid any kind of overflow.