Notes on Python cookbook 3rd (2.14): merging and splicing strings

Giant ship 2020-11-14 01:02:14
notes python cookbook 3rd rd

Merge concatenated strings


You want to combine several small strings into one big string


If the string you want to merge is in a sequence or iterable in , So the fastest way is to use join() Method . such as :

>>> parts = ['Is', 'Chicago', 'Not', 'Chicago?']
>>> ' '.join(parts)
'Is Chicago Not Chicago?'
>>> ','.join(parts)
>>> ''.join(parts)

First look , It looks strange , however join() A method specified as a string . Part of the reason for this is that the objects you want to connect to may come from a variety of different data sequences ( Such as the list , Tuples , Dictionaries , file , Set or generator, etc ), If you define one on all of these objects join() The method is obviously redundant . So you just need to specify the split string you want and call it join() Method to combine text fragments .

If you just merge a few strings , Use the plus sign (+) Usually enough :

>>> a = 'Is Chicago'
>>> b = 'Not Chicago?'
>>> a + ' ' + b
'Is Chicago Not Chicago?'

plus (+) The operator also works well as an alternative to some complex string formatting , such as :

>>> print('{} {}'.format(a,b))
Is Chicago Not Chicago?
>>> print(a + ' ' + b)
Is Chicago Not Chicago?

If you want to merge two literal strings in the source code , You just need to simply put them together , There's no need to use the plus sign (+). such as :

>>> a = 'Hello' 'World'
>>> a


The programmer usually brings serious performance loss to the application due to improper selection when formatting strings .

The most important thing to notice is , When we use the plus sign (+) Operators are very inefficient to join a large number of strings , Because plus connection will cause memory copy and garbage collection operation . Special , You should never write string concatenation code like this :

s = ''
for p in parts:
s += p

This way of writing is better than using join() The method runs slower , Because every execution += Operation will create a new string object . You'd better collect all the string fragments first and then connect them .

A relatively clever trick is to use generator expressions to convert data into strings while merging strings , such as :

>>> data = ['ACME', 50, 91.1]
>>> ','.join(str(d) for d in data)

Also pay attention to unnecessary string concatenation operations . Sometimes a programmer does not have to do a connection operation when it is unnecessary to do so . For example, when printing :

print(a + ':' + b + ':' + c) # Ugly
print(':'.join([a, b, c])) # Still ugly
print(a, b, c, sep=':') # Better

When mixed with I/O Operations and string concatenation operations , Sometimes you need to study your program carefully . such as , Consider the following two end code snippets :

# Version 1 (string concatenation)
f.write(chunk1 + chunk2)
# Version 2 (separate I/O operations)

If two strings are very small , Then the performance of the first version will be better , because I/O System calls are inherently slow . On the other hand , If two strings are large , So the second version might be more efficient , Because it avoids creating a large temporary result and copying a lot of memory block data . Or that sentence , Sometimes it's up to you to decide which solution to use based on the characteristics of your application .

Finally, let's talk about it , If you're going to write output code that builds a lot of small strings , You'd better consider using generator functions , utilize yield Statement produces output fragments . such as :

def sample():
yield 'Is'
yield 'Chicago'
yield 'Not'
yield 'Chicago?'

One interesting aspect of this approach is that it doesn't make assumptions about how the output fragment should be organized . for example , You can simply use join() Methods combine these fragments together :

text = ''.join(sample())

Or you can redirect string fragments to I/O:

for part in sample():

Or you can also write some combinations I/O A hybrid scheme of operations :

def combine(source, maxsize):
parts = []
size = 0
for part in source:
size += len(part)
if size > maxsize:
yield ''.join(parts)
parts = []
size = 0
yield ''.join(parts)

# Combined with file operation 
with open('filename', 'w') as f:
for part in combine(sample(), 32768):
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