python中string模块各属性以及函数的用法

任何语言都离不开字符，那就会涉及对字符的操作，尤其是脚本语言更是频繁，不管是生产环境还是面试考验都要面对字符串的操作。

python的字符串操作通过2部分的方法函数基本上就可以解决所有的字符串操作需求：

• python的字符串属性函数

• python的string模块

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1.字符串属性函数

系统版本：CentOS release 6.2 (Final)2.6.32-220.el6.x86_64

python版本：Python 2.6.6

字符串属性方法

字符串格式输出对齐

>>> str='stRINg lEArn' 
 
>>> 
 
>>> str.center(20)   #生成20个字符长度，str排中间 
 
'  stRINg lEArn  ' 
 
>>>  
 
>>> str.ljust(20)    #str左对齐 
 
'stRINg lEArn    '  
 
>>> 
 
>>> str.rjust(20)    #str右对齐 
 
'    stRINg lEArn' 
 
>>>  
 
>>> str.zfill(20)    #str右对齐，左边填充0 
 
'00000000stRINg lEArn'

大小写转换

>>> str='stRINg lEArn'  
 
>>>  
 
>>> str.upper() #转大写 
 
'STRING LEARN' 
 
>>>  
 
>>> str.lower() #转小写 
 
'string learn' 
 
>>>  
 
>>> str.capitalize() #字符串首为大写，其余小写 
 
'String learn' 
 
>>>  
 
>>> str.swapcase() #大小写对换 
 
'STrinG LeaRN' 
 
>>>  
 
>>> str.title() #以分隔符为标记，首字符为大写，其余为小写 
 
'String Learn'

字符串条件判断

>>> str='0123' 
 
>>> str.isalnum() #是否全是字母和数字，并至少有一个字符 
True 
 
>>> str.isdigit() #是否全是数字，并至少有一个字符 
True 
 
 
 
>>> str='abcd' 
 
>>> str.isalnum() 
 
True 
 
>>> str.isalpha() #是否全是字母，并至少有一个字符 
True 
 
>>> str.islower() #是否全是小写，当全是小写和数字一起时候，也判断为True 
True 
 
 
 
>>> str='abcd0123' 
 
>>> str.islower() #同上 
 
True 
 
>>> str.isalnum()  
 
True 
 
 
 
>>> str=' ' 
 
>>> str.isspace() #是否全是空白字符，并至少有一个字符 
True 
 
>>> str='ABC' 
 
>>> str.isupper() #是否全是大写，当全是大写和数字一起时候，也判断为True 
True 
 
>>> str='Abb Acc' 
 
>>> str.istitle() #所有单词字首都是大写，标题 
True 
 
 
>>> str='string learn' 
>>> str.startswith('str') #判断字符串以'str'开头 
True 
>>> str.endswith('arn') #判读字符串以'arn'结尾 
True

字符串搜索定位与替换

>>> str='string lEARn' 
 
>>> 
 
>>> str.find('a')  #查找字符串，没有则返回-1，有则返回查到到第一个匹配的索引 
 
-1 
 
>>> str.find('n') 
 
4 
 
>>> str.rfind('n')  #同上，只是返回的索引是最后一次匹配的 
 
11 
 
>>> 
 
>>> str.index('a')  #如果没有匹配则报错 
 
Traceback (most recent call last): 
 
 File "<stdin>", line 1, in <module> 
 
ValueError: substring not found 
 
>>> str.index('n')  #同find类似,返回第一次匹配的索引值 
 
4 
 
>>> str.rindex('n') #返回最后一次匹配的索引值 
 
11 
 
>>> 
 
>>> str.count('a')  #字符串中匹配的次数 
 
0 
 
>>> str.count('n')  #同上 
 
2 
 
>>> 
 
>>> str.replace('EAR','ear') #匹配替换 
 
'string learn' 
 
>>> str.replace('n','N') 
 
'striNg lEARN' 
 
>>> str.replace('n','N',1) 
 
'striNg lEARn' 
 
>>> 
 
>>> 
 
>>> str.strip('n') #删除字符串首尾匹配的字符，通常用于默认删除回车符 
 
'string lEAR' 
 
>>> str.lstrip('n') #左匹配 
 
'string lEARn' 
 
>>> str.rstrip('n') #右匹配 
 
'string lEAR' 
 
>>> 
 
>>> str=' tab' 
 
>>> str.expandtabs() #把制表符转为空格 
 
'  tab' 
 
>>> str.expandtabs(2) #指定空格数 
 
' tab'

字符串编码与解码

>>> str='字符串学习' 
 
>>> str 
 
'xe5xadx97xe7xacxa6xe4xb8xb2xe5xadxa6xe4xb9xa0' 
 
>>> 
 
>>> str.decode('utf-8')    #解码过程，将utf-8解码为unicode 
 
u'u5b57u7b26u4e32u5b66u4e60' 
 
 
 
>>> str.decode('utf-8').encode('gbk') #编码过程，将unicode编码为gbk 
 
'xd7xd6xb7xfbxb4xaexd1xa7xcfxb0' 
 
>>> str.decode('utf-8').encode('utf-8') #将unicode编码为utf-8 
 
'xe5xadx97xe7xacxa6xe4xb8xb2xe5xadxa6xe4xb9xa0'

字符串分割变换

>>> str='Learn string' 
 
>>> '-'.join(str) 
 
'L-e-a-r-n- -s-t-r-i-n-g' 
 
>>> l1=['Learn','string'] 
 
>>> '-'.join(l1) 
 
'Learn-string' 
 
>>> 
 
>>> str.split('n') 
 
['Lear', ' stri', 'g'] 
 
>>> str.split('n',1) 
 
['Lear', ' string'] 
 
>>> str.rsplit('n',1) 
 
['Learn stri', 'g'] 
 
>>> 
 
>>> str.splitlines() 
 
['Learn string'] 
 
>>> 
 
>>> str.partition('n') 
 
('Lear', 'n', ' string') 
 
>>> str.rpartition('n') 
 
('Learn stri', 'n', 'g')

string模块源代码

"""A collection of string operations (most are no longer used). 
 
 
Warning: most of the code you see here isn't normally used nowadays. 
 
Beginning with Python 1.6, many of these functions are implemented as 
 
methods on the standard string object. They used to be implemented by 
 
a built-in module called strop, but strop is now obsolete itself. 
 
 
Public module variables: 
 
 
whitespace -- a string containing all characters considered whitespace 
 
lowercase -- a string containing all characters considered lowercase letters 
 
uppercase -- a string containing all characters considered uppercase letters 
 
letters -- a string containing all characters considered letters 
 
digits -- a string containing all characters considered decimal digits 
 
hexdigits -- a string containing all characters considered hexadecimal digits 
 
octdigits -- a string containing all characters considered octal digits 
 
punctuation -- a string containing all characters considered punctuation 
 
printable -- a string containing all characters considered printable 
 
 
""" 
 
 
# Some strings for ctype-style character classification 
 
whitespace = ' tnrvf' 
 
lowercase = 'abcdefghijklmnopqrstuvwxyz' 
 
uppercase = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ' 
 
letters = lowercase + uppercase 
 
ascii_lowercase = lowercase 
 
ascii_uppercase = uppercase 
 
ascii_letters = ascii_lowercase + ascii_uppercase 
 
digits = '0123456789' 
 
hexdigits = digits + 'abcdef' + 'ABCDEF' 
 
octdigits = '01234567' 
 
punctuation = """!"#$%&'()*+,-./:;<=>?@[]^_`{|}~""" 
 
printable = digits + letters + punctuation + whitespace 
 
 
 
# Case conversion helpers 
 
# Use str to convert Unicode literal in case of -U 
 
l = map(chr, xrange(256)) 
 
_idmap = str('').join(l) 
 
del l 
 
 
 
# Functions which aren't available as string methods. 
 
 
 
# Capitalize the words in a string, e.g. " aBc dEf " -> "Abc Def". 
 
def capwords(s, sep=None): 
 
  """capwords(s [,sep]) -> string 
 
 
  Split the argument into words using split, capitalize each 
 
  word using capitalize, and join the capitalized words using 
 
  join. If the optional second argument sep is absent or None, 
 
  runs of whitespace characters are replaced by a single space 
 
  and leading and trailing whitespace are removed, otherwise 
 
  sep is used to split and join the words. 
 
 
  """ 
 
  return (sep or ' ').join(x.capitalize() for x in s.split(sep)) 
 
 
# Construct a translation string 
 
_idmapL = None 
 
def maketrans(fromstr, tostr): 
 
  """maketrans(frm, to) -> string 
 
 
  Return a translation table (a string of 256 bytes long) 
 
  suitable for use in string.translate. The strings frm and to 
 
  must be of the same length. 
 
 
  """ 
 
  if len(fromstr) != len(tostr): 
 
    raise ValueError, "maketrans arguments must have same length" 
 
  global _idmapL 
 
  if not _idmapL: 
 
    _idmapL = list(_idmap) 
 
  L = _idmapL[:] 
 
  fromstr = map(ord, fromstr) 
 
  for i in range(len(fromstr)): 
 
    L[fromstr[i]] = tostr[i] 
 
  return ''.join(L) 
 
 
 
#################################################################### 
 
import re as _re 
 
 
class _multimap: 
 
  """Helper class for combining multiple mappings. 
 
 
 
  Used by .{safe_,}substitute() to combine the mapping and keyword 
 
  arguments. 
 
  """ 
 
  def __init__(self, primary, secondary): 
 
    self._primary = primary 
 
    self._secondary = secondary 
 
 
 
  def __getitem__(self, key): 
 
    try: 
 
      return self._primary[key] 
 
    except KeyError: 
 
      return self._secondary[key] 
 
 
 
class _TemplateMetaclass(type): 
 
  pattern = r""" 
 
  %(delim)s(?: 
 
   (?P<escaped>%(delim)s) | # Escape sequence of two delimiters 
 
   (?P<named>%(id)s) | # delimiter and a Python identifier 
 
   {(?P<braced>%(id)s)} | # delimiter and a braced identifier 
 
   (?P<invalid>) # Other ill-formed delimiter exprs 
 
  ) 
 
  """ 
 
 
  def __init__(cls, name, bases, dct): 
 
    super(_TemplateMetaclass, cls).__init__(name, bases, dct) 
 
    if 'pattern' in dct: 
 
      pattern = cls.pattern 
 
    else: 
 
      pattern = _TemplateMetaclass.pattern % { 
 
        'delim' : _re.escape(cls.delimiter), 
 
        'id' : cls.idpattern, 
 
        } 
 
    cls.pattern = _re.compile(pattern, _re.IGNORECASE | _re.VERBOSE) 
 
 
 
class Template: 
 
  """A string class for supporting $-substitutions.""" 
 
  __metaclass__ = _TemplateMetaclass 
 
 
 
  delimiter = '$' 
 
  idpattern = r'[_a-z][_a-z0-9]*' 
 
 
 
  def __init__(self, template): 
 
    self.template = template 
 
 
 
  # Search for $$, $identifier, ${identifier}, and any bare $'s 
 
 
 
  def _invalid(self, mo): 
 
    i = mo.start('invalid') 
 
    lines = self.template[:i].splitlines(True) 
 
    if not lines: 
 
      colno = 1 
 
      lineno = 1 
 
    else: 
 
      colno = i - len(''.join(lines[:-1])) 
 
      lineno = len(lines) 
 
    raise ValueError('Invalid placeholder in string: line %d, col %d' % 
 
             (lineno, colno)) 
 
 
 
  def substitute(self, *args, **kws): 
 
    if len(args) > 1: 
 
      raise TypeError('Too many positional arguments') 
 
    if not args: 
 
      mapping = kws 
 
    elif kws: 
 
      mapping = _multimap(kws, args[0]) 
 
    else: 
 
      mapping = args[0] 
 
    # Helper function for .sub() 
 
    def convert(mo): 
 
      # Check the most common path first. 
 
      named = mo.group('named') or mo.group('braced') 
 
      if named is not None: 
 
        val = mapping[named] 
 
        # We use this idiom instead of str() because the latter will 
 
        # fail if val is a Unicode containing non-ASCII characters. 
 
        return '%s' % (val,) 
 
      if mo.group('escaped') is not None: 
 
        return self.delimiter 
 
      if mo.group('invalid') is not None: 
 
        self._invalid(mo) 
 
      raise ValueError('Unrecognized named group in pattern', 
 
               self.pattern) 
 
    return self.pattern.sub(convert, self.template) 
 
 
 
  def safe_substitute(self, *args, **kws): 
 
    if len(args) > 1: 
 
      raise TypeError('Too many positional arguments') 
 
    if not args: 
 
      mapping = kws 
 
    elif kws: 
 
      mapping = _multimap(kws, args[0]) 
 
    else: 
 
      mapping = args[0] 
 
    # Helper function for .sub() 
 
    def convert(mo): 
 
      named = mo.group('named') 
 
      if named is not None: 
 
        try: 
 
          # We use this idiom instead of str() because the latter 
 
          # will fail if val is a Unicode containing non-ASCII 
 
          return '%s' % (mapping[named],) 
 
        except KeyError: 
 
          return self.delimiter + named 
 
      braced = mo.group('braced') 
 
      if braced is not None: 
 
        try: 
 
          return '%s' % (mapping[braced],) 
 
        except KeyError: 
 
          return self.delimiter + '{' + braced + '}' 
 
      if mo.group('escaped') is not None: 
 
        return self.delimiter 
 
      if mo.group('invalid') is not None: 
 
        return self.delimiter 
 
      raise ValueError('Unrecognized named group in pattern', 
 
               self.pattern) 
 
    return self.pattern.sub(convert, self.template) 
 
 
 
 
 
 
 
#################################################################### 
 
# NOTE: Everything below here is deprecated. Use string methods instead. 
 
# This stuff will go away in Python 3.0. 
 
 
 
# Backward compatible names for exceptions 
 
index_error = ValueError 
 
atoi_error = ValueError 
 
atof_error = ValueError 
 
atol_error = ValueError 
 
 
 
# convert UPPER CASE letters to lower case 
 
def lower(s): 
 
  """lower(s) -> string 
 
 
 
  Return a copy of the string s converted to lowercase. 
 
 
 
  """ 
 
  return s.lower() 
 
 
 
# Convert lower case letters to UPPER CASE 
 
def upper(s): 
 
  """upper(s) -> string 
 
 
 
  Return a copy of the string s converted to uppercase. 
 
 
 
  """ 
 
  return s.upper() 
 
 
 
# Swap lower case letters and UPPER CASE 
 
def swapcase(s): 
 
  """swapcase(s) -> string 
 
 
 
  Return a copy of the string s with upper case characters 
 
  converted to lowercase and vice versa. 
 
 
 
  """ 
 
  return s.swapcase() 
 
 
 
# Strip leading and trailing tabs and spaces 
 
def strip(s, chars=None): 
 
  """strip(s [,chars]) -> string 
 
 
 
  Return a copy of the string s with leading and trailing 
 
  whitespace removed. 
 
  If chars is given and not None, remove characters in chars instead. 
 
  If chars is unicode, S will be converted to unicode before stripping. 
 
 
 
  """ 
 
  return s.strip(chars) 
 
 
 
# Strip leading tabs and spaces 
 
def lstrip(s, chars=None): 
 
  """lstrip(s [,chars]) -> string 
 
 
 
  Return a copy of the string s with leading whitespace removed. 
 
  If chars is given and not None, remove characters in chars instead. 
 
 
 
  """ 
 
  return s.lstrip(chars) 
 
 
 
# Strip trailing tabs and spaces 
 
def rstrip(s, chars=None): 
 
  """rstrip(s [,chars]) -> string 
 
 
 
  Return a copy of the string s with trailing whitespace removed. 
 
  If chars is given and not None, remove characters in chars instead. 
 
 
 
  """ 
 
  return s.rstrip(chars) 
 
 
 
 
 
# Split a string into a list of space/tab-separated words 
 
def split(s, sep=None, maxsplit=-1): 
 
  """split(s [,sep [,maxsplit]]) -> list of strings 
 
 
 
  Return a list of the words in the string s, using sep as the 
 
  delimiter string. If maxsplit is given, splits at no more than 
 
  maxsplit places (resulting in at most maxsplit+1 words). If sep 
 
  is not specified or is None, any whitespace string is a separator. 
 
 
 
  (split and splitfields are synonymous) 
 
 
 
  """ 
 
  return s.split(sep, maxsplit) 
 
splitfields = split 
 
 
 
# Split a string into a list of space/tab-separated words 
 
def rsplit(s, sep=None, maxsplit=-1): 
 
  """rsplit(s [,sep [,maxsplit]]) -> list of strings 
 
 
 
  Return a list of the words in the string s, using sep as the 
 
  delimiter string, starting at the end of the string and working 
 
  to the front. If maxsplit is given, at most maxsplit splits are 
 
  done. If sep is not specified or is None, any whitespace string 
 
  is a separator. 
 
  """ 
 
  return s.rsplit(sep, maxsplit) 
 
 
 
# Join fields with optional separator 
 
def join(words, sep = ' '): 
 
  """join(list [,sep]) -> string 
 
 
 
  Return a string composed of the words in list, with 
 
  intervening occurrences of sep. The default separator is a 
 
  single space. 
 
 
 
  (joinfields and join are synonymous) 
 
 
 
  """ 
 
  return sep.join(words) 
 
joinfields = join 
 
 
 
# Find substring, raise exception if not found 
 
def index(s, *args): 
 
  """index(s, sub [,start [,end]]) -> int 
 
 
 
  Like find but raises ValueError when the substring is not found. 
 
 
 
  """ 
 
  return s.index(*args) 
 
 
 
# Find last substring, raise exception if not found 
 
def rindex(s, *args): 
 
  """rindex(s, sub [,start [,end]]) -> int 
 
 
 
  Like rfind but raises ValueError when the substring is not found. 
 
 
 
  """ 
 
  return s.rindex(*args) 
 
 
 
# Count non-overlapping occurrences of substring 
 
def count(s, *args): 
 
  """count(s, sub[, start[,end]]) -> int 
 
 
 
  Return the number of occurrences of substring sub in string 
 
  s[start:end]. Optional arguments start and end are 
 
  interpreted as in slice notation. 
 
 
 
  """ 
 
  return s.count(*args) 
 
 
 
# Find substring, return -1 if not found 
 
def find(s, *args): 
 
  """find(s, sub [,start [,end]]) -> in 
 
 
 
  Return the lowest index in s where substring sub is found, 
 
  such that sub is contained within s[start,end]. Optional 
 
  arguments start and end are interpreted as in slice notation. 
 
 
 
  Return -1 on failure. 
 
 
 
  """ 
 
  return s.find(*args) 
 
 
 
# Find last substring, return -1 if not found 
 
def rfind(s, *args): 
 
  """rfind(s, sub [,start [,end]]) -> int 
 
 
 
  Return the highest index in s where substring sub is found, 
 
  such that sub is contained within s[start,end]. Optional 
 
  arguments start and end are interpreted as in slice notation. 
 
 
 
  Return -1 on failure. 
 
 
 
  """ 
 
  return s.rfind(*args) 
 
 
 
# for a bit of speed 
 
_float = float 
 
_int = int 
 
_long = long 
 
 
 
# Convert string to float 
 
def atof(s): 
 
  """atof(s) -> float 
 
 
 
  Return the floating point number represented by the string s. 
 
 
 
  """ 
 
  return _float(s) 
 
 
 
 
 
# Convert string to integer 
 
def atoi(s , base=10): 
 
  """atoi(s [,base]) -> int 
 
 
 
  Return the integer represented by the string s in the given 
 
  base, which defaults to 10. The string s must consist of one 
 
  or more digits, possibly preceded by a sign. If base is 0, it 
 
  is chosen from the leading characters of s, 0 for octal, 0x or 
 
  0X for hexadecimal. If base is 16, a preceding 0x or 0X is 
 
  accepted. 
 
 
 
  """ 
 
  return _int(s, base) 
 
 
 
 
 
# Convert string to long integer 
 
def atol(s, base=10): 
 
  """atol(s [,base]) -> long 
 
 
 
  Return the long integer represented by the string s in the 
 
  given base, which defaults to 10. The string s must consist 
 
  of one or more digits, possibly preceded by a sign. If base 
 
  is 0, it is chosen from the leading characters of s, 0 for 
 
  octal, 0x or 0X for hexadecimal. If base is 16, a preceding 
 
  0x or 0X is accepted. A trailing L or l is not accepted, 
 
  unless base is 0. 
 
 
 
  """ 
 
  return _long(s, base) 
 
 
 
 
 
# Left-justify a string 
 
def ljust(s, width, *args): 
 
  """ljust(s, width[, fillchar]) -> string 
 
 
 
  Return a left-justified version of s, in a field of the 
 
  specified width, padded with spaces as needed. The string is 
 
  never truncated. If specified the fillchar is used instead of spaces. 
 
 
 
  """ 
 
  return s.ljust(width, *args) 
 
 
 
# Right-justify a string 
 
def rjust(s, width, *args): 
 
  """rjust(s, width[, fillchar]) -> string 
 
 
 
  Return a right-justified version of s, in a field of the 
 
  specified width, padded with spaces as needed. The string is 
 
  never truncated. If specified the fillchar is used instead of spaces. 
 
 
 
  """ 
 
  return s.rjust(width, *args) 
 
 
 
# Center a string 
 
def center(s, width, *args): 
 
  """center(s, width[, fillchar]) -> string 
 
 
 
  Return a center version of s, in a field of the specified 
 
  width. padded with spaces as needed. The string is never 
 
  truncated. If specified the fillchar is used instead of spaces. 
 
 
 
  """ 
 
  return s.center(width, *args) 
 
 
 
# Zero-fill a number, e.g., (12, 3) --> '012' and (-3, 3) --> '-03' 
 
# Decadent feature: the argument may be a string or a number 
 
# (Use of this is deprecated; it should be a string as with ljust c.s.) 
 
def zfill(x, width): 
 
  """zfill(x, width) -> string 
 
 
 
  Pad a numeric string x with zeros on the left, to fill a field 
 
  of the specified width. The string x is never truncated. 
 
 
 
  """ 
 
  if not isinstance(x, basestring): 
 
    x = repr(x) 
 
  return x.zfill(width) 
 
 
 
# Expand tabs in a string. 
 
# Doesn't take non-printing chars into account, but does understand n. 
 
def expandtabs(s, tabsize=8): 
 
  """expandtabs(s [,tabsize]) -> string 
 
 
 
  Return a copy of the string s with all tab characters replaced 
 
  by the appropriate number of spaces, depending on the current 
 
  column, and the tabsize (default 8). 
 
 
 
  """ 
 
  return s.expandtabs(tabsize) 
 
 
 
# Character translation through look-up table. 
 
def translate(s, table, deletions=""): 
 
  """translate(s,table [,deletions]) -> string 
 
 
 
  Return a copy of the string s, where all characters occurring 
 
  in the optional argument deletions are removed, and the 
 
  remaining characters have been mapped through the given 
 
  translation table, which must be a string of length 256. The 
 
  deletions argument is not allowed for Unicode strings. 
 
 
 
  """ 
 
  if deletions or table is None: 
 
    return s.translate(table, deletions) 
 
  else: 
 
    # Add s[:0] so that if s is Unicode and table is an 8-bit string, 
 
    # table is converted to Unicode. This means that table *cannot* 
 
    # be a dictionary -- for that feature, use u.translate() directly. 
 
    return s.translate(table + s[:0]) 
 
 
 
# Capitalize a string, e.g. "aBc dEf" -> "Abc def". 
 
def capitalize(s): 
 
  """capitalize(s) -> string 
 
 
 
  Return a copy of the string s with only its first character 
 
  capitalized. 
 
 
 
  """ 
 
  return s.capitalize() 
 
 
 
# Substring replacement (global) 
 
def replace(s, old, new, maxsplit=-1): 
 
  """replace (str, old, new[, maxsplit]) -> string 
 
 
 
  Return a copy of string str with all occurrences of substring 
 
  old replaced by new. If the optional argument maxsplit is 
 
  given, only the first maxsplit occurrences are replaced. 
 
 
 
  """ 
 
  return s.replace(old, new, maxsplit) 
 
 
 
 
 
# Try importing optional built-in module "strop" -- if it exists, 
 
# it redefines some string operations that are 100-1000 times faster. 
 
# It also defines values for whitespace, lowercase and uppercase 
 
# that match <ctype.h>'s definitions. 
 
 
 
try: 
 
  from strop import maketrans, lowercase, uppercase, whitespace 
 
  letters = lowercase + uppercase 
 
except ImportError: 
 
  pass # Use the original versions 
 
 
 
######################################################################## 
 
# the Formatter class 
 
# see PEP 3101 for details and purpose of this class 
 
 
 
# The hard parts are reused from the C implementation. They're exposed as "_" 
 
# prefixed methods of str and unicode. 
 
 
 
# The overall parser is implemented in str._formatter_parser. 
 
# The field name parser is implemented in str._formatter_field_name_split 
 
 
 
class Formatter(object): 
 
  def format(self, format_string, *args, **kwargs): 
 
    return self.vformat(format_string, args, kwargs) 
 
 
 
  def vformat(self, format_string, args, kwargs): 
 
    used_args = set() 
 
    result = self._vformat(format_string, args, kwargs, used_args, 2) 
 
    self.check_unused_args(used_args, args, kwargs) 
 
    return result 
 
 
 
  def _vformat(self, format_string, args, kwargs, used_args, recursion_depth): 
 
    if recursion_depth < 0: 
 
      raise ValueError('Max string recursion exceeded') 
 
    result = [] 
 
    for literal_text, field_name, format_spec, conversion in  
 
        self.parse(format_string): 
 
 
 
      # output the literal text 
 
      if literal_text: 
 
        result.append(literal_text) 
 
 
 
      # if there's a field, output it 
 
      if field_name is not None: 
 
        # this is some markup, find the object and do 
 
        # the formatting 
 
 
 
        # given the field_name, find the object it references 
 
        # and the argument it came from 
 
        obj, arg_used = self.get_field(field_name, args, kwargs) 
 
        used_args.add(arg_used) 
 
 
 
        # do any conversion on the resulting object 
 
        obj = self.convert_field(obj, conversion) 
 
 
 
        # expand the format spec, if needed 
 
        format_spec = self._vformat(format_spec, args, kwargs, 
 
                      used_args, recursion_depth-1) 
 
 
 
        # format the object and append to the result 
 
        result.append(self.format_field(obj, format_spec)) 
 
 
 
    return ''.join(result) 
 
 
 
 
 
  def get_value(self, key, args, kwargs): 
 
    if isinstance(key, (int, long)): 
 
      return args[key] 
 
    else: 
 
      return kwargs[key] 
 
 
 
 
 
  def check_unused_args(self, used_args, args, kwargs): 
 
    pass 
 
 
 
 
 
  def format_field(self, value, format_spec): 
 
    return format(value, format_spec) 
 
 
 
 
 
  def convert_field(self, value, conversion): 
 
    # do any conversion on the resulting object 
 
    if conversion == 'r': 
 
      return repr(value) 
 
    elif conversion == 's': 
 
      return str(value) 
 
    elif conversion is None: 
 
      return value 
 
    raise ValueError("Unknown converion specifier {0!s}".format(conversion)) 
 
 
 
 
 
  # returns an iterable that contains tuples of the form: 
 
  # (literal_text, field_name, format_spec, conversion) 
 
  # literal_text can be zero length 
 
  # field_name can be None, in which case there's no 
 
  # object to format and output 
 
  # if field_name is not None, it is looked up, formatted 
 
  # with format_spec and conversion and then used 
 
  def parse(self, format_string): 
 
    return format_string._formatter_parser() 
 
 
 
 
 
  # given a field_name, find the object it references. 
 
  # field_name: the field being looked up, e.g. "0.name" 
 
  # or "lookup[3]" 
 
  # used_args: a set of which args have been used 
 
  # args, kwargs: as passed in to vformat 
 
  def get_field(self, field_name, args, kwargs): 
 
    first, rest = field_name._formatter_field_name_split() 
 
 
 
    obj = self.get_value(first, args, kwargs) 
 
 
 
    # loop through the rest of the field_name, doing 
 
    # getattr or getitem as needed 
 
    for is_attr, i in rest: 
 
      if is_attr: 
 
        obj = getattr(obj, i) 
 
      else: 
 
        obj = obj[i] 
 
 
 
    return obj, first

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