Miscellaneous String Methods

Documentation

• Text Sequence Type — str

Batteries Included: Checks

Lots of small checks (returning boolean) - for example …

• '...'.isspace(): contains only whitespace

• Character types

  • '...'.isalpha()

  • '...'.isalnum()

  • '...'.isdigit()

• Case tests

  • '...'.isupper()

  • '...'.islower()

• '...'.isidentifier(): a valid python identifier (e.g. variable name)

• Lots of others ⟶ save work and RTFM prior to coding

Batteries Included: Search

Substring search …

• '...'.count(s): number of occurences of s

• '...'.startswith(s), .endswith(s)

• '...'.find(sub[, start[, end]]): find sub, starting at start (default 0), ending at end (default len())

  • end is exclusive ⟶ '...'[start:end]

  • Returns index, or -1 if not found

• '...'.index(sub[, start[, end]]): like find, but raises exception if not found

• '...'.rfind(sub[, start[, end]]): from the end

• '...'.rindex(sub[, start[, end]]): from the end

Substring Search: Examples

'/etc/passwd'.startswith('/etc/')

True

'notes.txt'.endswith('.txt')

True

'this is a thistle with many thorns'.count('th')

4

'haystack containing needle and straw'.find('needle')

20

• find() returns position - -1 if not found

'haystack containing needle and straw'.find('mouse')

-1

• index() returns position - raises if not found

'haystack containing needle and straw'.index('mouse')

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[6], line 1
----> 1 'haystack containing needle and straw'.index('mouse')

ValueError: substring not found

Split and Join (1)

• Very common operations

• Error prone ⟶ writing them is a major annoyance

• Off-by-one errors

'one:two:three'.split(':')

['one', 'two', 'three']

':'.join(['one', 'two', 'three'])

'one:two:three'

':'.join([])

''

':'.join(['one'])

'one'

Split and Join (2)

• Split at most 2 fields

  'one:two:three:four'.split(':', 2)
  

  ['one', 'two', 'three:four']
  

  'one:two:three:four'.rsplit(':', 2)
  

  ['one:two', 'three', 'four']
  

• Real life example: /etc/passwd

  username,rest = 'jfasch:x:1000:...'.split(':', 1)
  username
  

  'jfasch'
  

  rest
  

  'x:1000:...'