Python:Strings
Strings are immutable, all methods return a new string
Basics
- str1 + str2
- Return concatenation of str1 and str2
- str1 += str2
- Append str2 to str1
- str1 * 3
- Return str1 3 times
Formatting
Basic
- str1.replace(old,new[,cnt])
- Return str1 with old replaced by new (cnt times).
- str1.strip(<chars>)
- Return str1 with all trailing and leading <chars> removed. If <chars> is omitted all trailing and leading whitespaces are removed.
- str1.rstrip('\r\n')
- str1.lstrip('<char>')
- Return str1 with all newline characters (windows, mac or unix) stripped from the end of str1, like perl 'chomp' does.
- lstrip removes characters from the beginning of str1
- Without character specification all whitespaces are removed.
- str1.upper() str1.lower() str1.title()
- Return str1 in upppercase, lowercase or with only all first characters in uppercase
- str1.join(list)
- str1.join(map(str,list))
- str1.join(str(e) for e in list)
- Join list (or set or other sequence) into a string with str1 as separator. The second and third form makes sure all elements are converted to string before they are joined.
- str1.split(sep[,max])
- Split string into a list on sep into max + 1 elements (remainder is put in last element)
- str1.splitlines([keepends])
- Split on newline, with 'keepends' the newline is preserved.
- str1.center(w)
- str1.ljust(w)
- str1.rjust(w)
- Put spaces around str1 until length 'w' is reached.
- str1.expandtabs(size)
- Replace tabs by 'size' number of spaces.
Using format
- str1.format(values)
- Fill in 'values' in str1-fields ({}). By numbering the fields they can be in a different order than the values.
- If values are in a dict, they can be addressed by their key.
Code Example
"Value 1: {}, Value2: {}".format(1,2)
"Value 2: {1}, Value1: {0}".format(1,2)
dict1 = {'value1':1, 'value2':2}
"Value 2: {value2}, Value1: {value1}".format(**dict1)
list1 = {'value1', 'value2'}
"Value 1: {}, Value2: {}".format(*list1)
version=3.6
f"Format strings exist since version {version}"
NOTE: Using format-stings (f"string with {variable}") you do not need .format anymore.
fstring = 'f"Convert a string to a format-string'
compiled_fstring = compile(fstring,fstring, 'eval')
print(eval(compiled_fstring))
- {[field]:formatspec}
- The format can be specified after the (optional) fieldnumber.
- [[fill]align][sign][#][0][width][grouping_option][.precision][type]
- Generic format specification. Anything not needed can be left out [1].
- e.g. "{:07d}".format(5) fill out with 0 in front to 7 digits -> '0000005'
- "{:010.6f}".format(5.7647) floating point with precision 6 and total width 10 left fill with 0 -> '005.764700'
- "{:10,.2f}".format(12345.7647) floating point with precision 2,total width 10 and comma as thousand separator -> ' 12,345.76'
- "{:#=+15_.3f}".format(10000) # as padding, padding after sign, always show sign, _ as thousand separator, floating point with precision 3 -> +####10_000.000
< | Left |
> | Right |
^ | Center |
= | Padding (after sign) |
# | Prepend for x, o and b types |
s | String |
c | Character |
d | decimal |
f | Float |
% | Percent |
o | Octal |
x | Hexadecimal |
b | Binary |
e | Exponent |
g | Python chooses between decimal, float or exponent |
Datastructures
Below can be used on datatypes like lists and tuples too, not on sets.
- json.dumps(dict, indent=4)
- Convert a dict into a json string nicely formatted. Indent each level with 4 spaces.
pprint provides similar functionality
Searching
Basic
- if <search> in str1
- True if <search> is in str1
- str1.count(<search>)
- Return how many times <search> is in str1
- str1.find(<search>)
- str1.index(<search>)
- Return where <search> is found in str1. If not found -1 with find, throw exception with index.
- str1.endswith(<search>)
- str.startswith(<search>)
- Return True if str1 ends/starts with <search> (else returns False).
Regular Expressions (regexp)
- import re
- The re modules provides Perl-like Regular Expressions matching for string and byte objects. The re module is standard available (no installation needed).
- re1 = re.compile(regexp)
- Create regular expression object to use for matching. This is more efficient if the regular expression in used several times in a program.
NOTE: In all examples below regexp can be replaced by the object from re.compile
- re.sub(regexp,new,str1)
- Return str1 with all parts matching regexp replaced with new.
- NOTE: str1 remains unchanged.
- NOTE2: re.sub is much more expensive than string.replace
- NOTE3: if you want to use search modifiers like DOTALL you have to use the object from re.compile.
- re.split(regexp,str1,max)
- Split str1 into a list on regexp, like with split above superfluous elements will be in the max + 1 element.
- match = re1.match(str1)
- match = re.match(regexp,str1)
- Find 'regexp' at the beginning of 'str1'. Return match object if found, else return None-object
- match = re.search(regexp,str1)
- Find first occurrence of 'regexp' in 'str1'. Return match object if found, else return None-object
- matchlist = re.findall(regexp,str1)
- Find all occurrences of 'regexp' in 'str1'. Return a list of strings.
- NOTE: When using a subexpression findall returns a list of tuples for the subexpression matches.
- Use finditer or, in this example to get the first subexpression matched (group(1)):
ListOfTuples = re.findall('(regexppart1 (subexpression1) regexppart2)',str1)
lst1 = [ a for a,b in ListOfTuples ]
- matches = re.finditer(regexp,str1)
- Find all occurrences of 'regexp' in 'str1'. Return a list of match objects.
Match Objects
- match .group()
- match .group(0)
- The matched string in match object 'mo'
- match .group(1)
- First submatch in the matched string in 'mo'. The first match is the first ( in the expression.
- match .start()
- The start position of the matched string in 'mo'
- match .end()
- The end position of the matched string in 'mo'
- match .span()
- Tuple with start and end position of the matched string in 'mo'
Search Modifieres
- re1 = re.search(regexp,str1,modifier)
- re1 = re.compile(regexp,modifier)
- Modify how matching is done
- re.DOTALL
- The . matches all characters (default is all characters except newline). Use for searching in web or book pages.
- re.I
- Ignore case
- re.M
- Multiline mode, ^ matches all line beginnings and $ all line endings.
Code Example:
import re
str1 = "The thing to cut in pieces"
rel1 = re.compile('h.*n')
print "Matching"
match = rel1.match(str1)
if match:
print match.group()
print match.start()
print match.end()
print match.span()
else:
print "no match at beginning of string"
print
print "Searching"
match = re.search('t.*n',str1)
if match:
print match.group()
print match.start()
print match.end()
print match.span()
print "Searching case insensitive"
match = re.search('h.*n',str1,re.I)
if match:
print match.group()
print match.start()
print match.end()
print match.span()
print "findall"
re1 = re.compile('t')
matchlist = re1.findall(str1)
for str2 in matchlist:
print str2
print
print "finditer"
re1 = re.compile('i.')
matches = re1.finditer(str1)
if matches:
for match in matches:
print match.group()
print match.start()
print match.end()
print match.span()
print