The most important Python object we'll deal with are strings. This tutorial presents a quick overview of Python string methods. However, let's first learn some very basics.
Python Strings - Basic Rules
- Strings may be enclosed by single or double quotes if they don't contain any line breaks;
- Strings may always be enclosed by 3 single or double quotes;
- A string enclosed by single quotes may also contain single quotes if we escape them with a backslash (\).
There's some more rules but these 3 will do for practical purposes. The 3 examples below demonstrate them.
Create Empty Python String
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*Create empty string object - single line. begin program. myString = '' #Create empty string print myString #(Empty line) print type(myString) #<type 'str'> end program. *Create multiple line string. begin program. myString = ''' T-TEST GROUPS=gender(0 1) /MISSING=ANALYSIS /VARIABLES=amount_spent /CRITERIA=CI(.95). ''' print myString end program. *Escape single quote in string. begin program. myString = 'I don\'t know!' print myString #I don't know! end program. |
Note: creating an empty string sounds pretty useless. However, we can concatenate lines of SPSS syntax or variable names to our empty string and this is often a nice way to build our syntax.
Overview Python String Methods
| What? | How? | Example | Returns |
|---|---|---|---|
| Extract Substring | [] | myString[0] | String |
| Concatenate 2(+) Strings | + or += | myString + myString | String |
| Find Leftmost Occurrence of Substring | find | myString.find('a') | Integer |
| Find Rightmost Occurrence of Substring | rfind | myString.rfind('a') | Integer |
| Replace 1(+) Characters | replace | myString.replace('a','b') | String |
| Find Length of String | len | len(myString) | Integer |
| Lowercase String | lower | myString.lower() | String |
| Uppercase String | upper | myString.upper() | String |
| Capitalize String | capitalize | myString.capitalize() | String |
| Remove Characters from Left Part of String | lstrip() | myString.lstrip() | String |
| Remove Characters from Right Part of String | rstrip() | myString.rstrip() | String |
| Remove Characters from Left and Right Part of String | strip() | myString.strip() | String |
| Convert String to Integer | int | int(myString) | Integer |
| Split String into Python List | split | myString.split(' ') | List |
| Check if String Starts With... | startswith | myString.startswith("var") | Boolean |
| Check if String Ends With... | startswith | myString.startswith("var") | Boolean |
| Left Pad String with Zeroes | zfill | myString.zfill(3) | String |
Extract Substring in Python
We extract substrings in Python with square brackets that may contain one or two indices and a colon. Like so,
myString[0]extracts the first character;myString[1:]extracts the second through last characters;myString[:4]extracts the first through fourth characters;myString[1:3]extracts the second through third characters;myString[-1]extracts the last character.
Python Substring Examples
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*SPSS Python substring examples. begin program. myString = 'abcdefghij' print myString[0] #a print myString[1:] #bcdefghij print myString[:4] #abcd print myString[1:3] #bc print myString[-1] #j end program. |
Concatenate 2(+) Strings
Basically, + concatenates two or more strings. Additionally, myString += 'a' is a nice shorthand for myString = myString + 'a' that we'll often use for building SPSS syntax.
Python Concatenate Examples
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*1. Concatenate with "+". begin program. myString = 'abc' print myString + 'def' #abcdef end program. *2. Concatenate with "+=" begin program. myString = 'abc' for i in range(5): myString += str(i) print myString #abc01234 end program. |
Find Leftmost Occurrence of Substring
Retrieving positions for single or multiple character substrings is done in Python with find. Keep in mind here that
- Python is fully case sensitive and
- Python objects are zero-indexed.
Like so, the indices for the characters in our examples are shown below.
Python Find Examples
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*Find leftmost occurrence of substring. begin program. myString = 'Cycling in the mountains is fun.' print myString.find('c') # 2 print myString.find('in') # 4 end program. |
Find Rightmost Occurrence of Substring
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*Find rightmost occurrence of substring. begin program. myString = 'Cycling in the mountains is fun.' print myString.rfind('i') # 25 print myString.rfind('in') # 21 end program. |
Replace 1(+) Characters
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*Replace one or more characters in string. begin program. myString = 'The cat caught the mouse in the living room.' print myString.replace('a','') #The ct cught the mouse in the living room. print myString.replace('the','a') # The cat caught a mouse in a living room. end program. |
Note: in line 5 we replace all a’s with an empty string. That is, we'll remove all a’s from our example sentence.
Find Length of String
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*Find length of string. begin program. myString = 'abcde' print len(myString) # 5 end program. |
Lowercase String
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*Lowercase string. begin program. myString = 'SPSS Is Fun!' print myString.lower() # spss is fun! end program. |
Uppercase String
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*Uppercase string. begin program. myString = 'This is Some Title' print myString.upper() # THIS IS SOME TITLE end program. |
Capitalize String
In Python, “capitalizing” means returning a string with its first character in upper case and all other characters in lower case -even if they were upper case in the original string.
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*Capitalize string. begin program. myString = 'aBcDeF' print myString.capitalize() # Abcdef end program. |
Remove Characters from Left Part of String
In Python, just lstrip() removes all spaces and tabs from the beginning of a string. Any other leading character can be removed by specifying it within the parentheses (line 12 below).
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*A. Remove whitespace from start of string. begin program. myString = ' left padding removed' print myString.lstrip() # left padding removed end program. *B. Remove asterisks (*) from start of string. begin program. myString = '****left padding removed' print myString.lstrip('*') # left padding removed end program. |
Remove Characters from Right Part of String
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*A. Remove whitespace from end of string. begin program. myString = 'right padding removed ' print myString.rstrip() # right padding removed end program. *B. Remove asterisks (*) from end of string. begin program. myString = 'right padding removed****' print myString.rstrip('*') # right padding removed end program. |
Remove Characters from Left and Right Part of String
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*A. Remove whitespace from end of string. begin program. myString = ' left and right padding removed ' print myString.strip() # left and right padding removed end program. *B. Remove asterisks (*) from end of string. begin program. myString = '****left and right padding removed****' print myString.rstrip('*') # left and right padding removed end program. |
Convert String to Integer
In Python, int converts a string to an integer. If a string contains anything else than digits, it'll crash with an error.
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*Convert String to Integer. begin program. myString = '123' myInt = int(myString) print type(myInt) # <type 'int'> print myInt # 123 end program. |
Split String into Python List
The example below splits a string into a Python list object. split always requires some separator. Splitting a string without any separator can be done with a list comprehension (line 14 below).
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*A. Split string into Python list object. begin program. myString = 'A A C A B C' myList = myString.split(' ') print type(myList) # <type 'list'> print myList # ['A', 'A', 'C', 'A', 'B', 'C'] end program. *B. Split string into Python list without separator. begin program. myString = 'AACABC' myList = [i for i in myString] print myList # ['A', 'A', 'C', 'A', 'B', 'C'] end program. |
Check if String Starts With...
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*Evaluate if string starts with given substring. begin program. myString = 'abcdef' print myString.startswith('abc') # True print myString.startswith('bcd') # False end program. *Typical use of startswith(). begin program. if myString.startswith('a'): print "First character is 'a'." else: print "First character is not 'a'." end program. |
Note: True and False are known as Boolean values. We mostly use them when we only want to run one or Python if statements.
Check if String Ends With...
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*Evaluate if string ends with given substring. begin program. myString = 'abcdef' print myString.endswith('f') # True print myString.endswith('e') # False end program. |
Left Pad String with Zeroes
In Python, zfill(3) left pads a string with zeroes up to a total length of 3 characters. We mostly do so when we want to sort numbers alphabetically: 002 comes before 010.
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*Left pad string with zeroes. begin program. myString = '1' print myString.zfill(3) # 001 myString = '10' print myString.zfill(3) # 010 end program. |
Python – The 5 Things You Want to Know
SPSS Python Text Replacement Tutorial
