Pyparsing a field that may or may not contain a value

The easiest way to parse text like "A: valueA B: valueB C: valueC" is to use the pyparsing SkipTo class:

a_expr = "A:" + SkipTo("B:")
b_expr = "B:" + SkipTo("C:")
c_expr = "C:" + SkipTo(LineEnd())
line_parser = a_expr + b_expr + c_expr

      

        
        
        
      

    

I would like to enlarge this a bit more:

• add a parse action to strip off leading and trailing spaces

• add the result name so it is easy to get the results after parsing the string

This is what this simple parser looks like:

NL = LineEnd()
a_expr = "A:" + SkipTo("B:").addParseAction(lambda t: [t[0].strip()])('A')
b_expr = "B:" + SkipTo("C:").addParseAction(lambda t: [t[0].strip()])('B')
c_expr = "C:" + SkipTo(NL).addParseAction(lambda t: [t[0].strip()])('C')
line_parser = a_expr + b_expr + c_expr

line_parser.runTests("""
    A: 100 B: Fred C:
    A:  B: a value with spaces C: 42
""")

      

        
        
        
      

    

gives:

 A: 100 B: Fred C:
['A:', '100', 'B:', 'Fred', 'C:', '']
- A: '100'
- B: 'Fred'
- C: ''


A:  B: a value with spaces C: 42
['A:', '', 'B:', 'a value with spaces', 'C:', '42']
- A: ''
- B: 'a value with spaces'
- C: '42'

      

        
        
        
      

    

I try to avoid copying / pasting code when I can, and rather automate "A followed by B" and "C follows end of line" with a list describing different prompt lines, then traversing that list to create each sub expression:

import pyparsing as pp

def make_prompt_expr(s):
    '''Define the expression for prompts as 'ABC:' '''
    return pp.Combine(pp.Literal(s) + ':')

def make_field_value_expr(next_expr):
    '''Define the expression for the field value as SkipTo(what comes next)'''
    return pp.SkipTo(next_expr).addParseAction(lambda t: [t[0].strip()])

def make_name(s):
    '''Convert prompt string to identifier form for results names'''
    return ''.join(s.split()).replace('-','_')

# use split to easily define list of prompts in order - makes it easy to update later if new prompts are added
prompts = "Capture MICR - Serial/Pos44/Trrt/Acct/Tc/Opt4/Split".split('/')

# keep a list of all the prompt-value expressions
exprs = []

# get a list of this-prompt, next-prompt pairs
for this_, next_ in zip(prompts, prompts[1:]  + [None]):
    field_name = make_name(this_)
    if next_ is not None:
        next_expr = make_prompt_expr(next_)
    else:
        next_expr = pp.LineEnd()

    # define the prompt-value expression for the current prompt string and add to exprs
    this_expr = make_prompt_expr(this_) + make_field_value_expr(next_expr)(field_name)
    exprs.append(this_expr)

# define a line parser as the And of all of the generated exprs
line_parser = pp.And(exprs)

line_parser.runTests("""\
Capture MICR - Serial:                  Pos44:  Trrt: 32904  Acct:        Tc:   2064        Opt4:          Split:
Capture MICR - Serial:  1729XYZ                Pos44:  Trrt: 32904  Acct:        Tc:   2064        Opt4: XXL         Split: 50
""")

      

        
        
        
      

    

gives:

Capture MICR - Serial:                  Pos44:  Trrt: 32904  Acct:        Tc:   2064        Opt4:          Split:
['Capture MICR - Serial:', '', 'Pos44:', '', 'Trrt:', '32904', 'Acct:', '', 'Tc:', '2064', 'Opt4:', '', 'Split:', '']
- Acct: ''
- CaptureMICR_Serial: ''
- Opt4: ''
- Pos44: ''
- Split: ''
- Tc: '2064'
- Trrt: '32904'


Capture MICR - Serial:  1729XYZ                Pos44:  Trrt: 32904  Acct:        Tc:   2064        Opt4: XXL         Split: 50
['Capture MICR - Serial:', '1729XYZ', 'Pos44:', '', 'Trrt:', '32904', 'Acct:', '', 'Tc:', '2064', 'Opt4:', 'XXL', 'Split:', '50']
- Acct: ''
- CaptureMICR_Serial: '1729XYZ'
- Opt4: 'XXL'
- Pos44: ''
- Split: '50'
- Tc: '2064'
- Trrt: '32904'