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How can I efficiently parse fixed-width file lines in Python?
How can I efficiently parse fixed-width file lines in Python?
Oct 30, 2024 pm 05:09 PM
Fast Parsing of Fixed Width File Lines
Parsing fixed width files, where each column occupies a specific number of characters in a line, can be a task requiring efficiency. Here's a discussion on how to achieve this efficiently:
The Problem
Consider a fixed-width file where the first 20 characters represent one column, followed by 21-30 for the second, and so on. Given a line with 100 characters, how can we effectively parse it into its respective columns?
Solutions
1. Struct Module:
Utilizing the Python standard library's struct module provides both simplicity and speed due to its C implementation. The code below demonstrates its usage:
<code class="python">import struct
fieldwidths = (2, -10, 24)
fmtstring = ' '.join('{}{}'.format(abs(fw), 'x' if fw < 0 else 's') for fw in fieldwidths)
# Convert Unicode input to bytes and decode result.
unpack = struct.Struct(fmtstring).unpack_from # Alias.
parse = lambda line: tuple(s.decode() for s in unpack(line.encode()))
# Parse a sample line.
line = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789\n'
fields = parse(line)
print('fields:', fields)</code>
Output:
fmtstring: '2s 10x 24s', record size: 36 chars
fields: ('AB', 'MNOPQRSTUVWXYZ0123456789')
2. Optimized String Slicing:
While string slicing is commonly used, it can become cumbersome for large lines. Here's an optimized approach:
<code class="python">from itertools import zip_longest
from itertools import accumulate
def make_parser(fieldwidths):
# Calculate slice boundaries.
cuts = tuple(cut for cut in accumulate(abs(fw) for fw in fieldwidths))
# Create field slice tuples.
flds = tuple(zip_longest(cuts, (0,)+cuts))[:-1] # Ignore final value.
# Construct the parsing function.
parse = lambda line: tuple(line[i:j] for i, j in flds)
parse.size = sum(abs(fw) for fw in fieldwidths)
parse.fmtstring = ' '.join('{}{}'.format(abs(fw), 'x' if fw < 0 else 's')
for fw in fieldwidths)
return parse
# Parse a sample line.
line = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789\n'
fieldwidths = (2, -10, 24) # Negative values indicate ignored padding fields.
parse = make_parser(fieldwidths)
fields = parse(line)
print('fmtstring:', parse.fmtstring, ', record size:', parse.size, 'chars')
print('fields:', fields)</code>
Output:
fmtstring: '2s 10x 24s', record size: 36 chars
fields: ('AB', 'MNOPQRSTUVWXYZ0123456789')The above is the detailed content of How can I efficiently parse fixed-width file lines in Python?. For more information, please follow other related articles on the PHP Chinese website!
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