Python3 f string vs .replace [duplicate] - python-3.x

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advanced string formatting vs template strings
(6 answers)
Closed 2 years ago.
I have a text file where most of the text is boilerplate, with around two dozen variables that I'm changing in Python depending on the room that the file pertains to. Which method of replacing the text is "better", wrapping the entire text file into one big triple quoted f'string', or stacking up a bunch of .replace() ?
The file isn't very big and there's only about 300 rooms, so in my case milliseconds don't really matter. I'm thinking that for readability and future edits the .replace() way would be better, but I don't want to create a bad habit if doing it that way is a bad idea. Thanks in advance for any help.
simplified pseudo code:
class Thing:
def __init__(self, name, var1, var2, var3):
self.name = name
self.var1 = var1
self.var2 = var2
self.var3 = var3
def doing_it_the_replace_way(thing):
with open('template.txt', 'r') as file:
file_data = file.read()
file_data = file_data.replace('placeholder_name', 'name')
file_data = file_data.replace('placeholder1', 'var1')
file_data = file_data.replace('placeholder2', 'var2')
file_data = file_data.replace('placeholder3', 'var3') # etc.
with open('output file.txt', 'w') as file:
file.write(file_data)
def doing_it_the_f_string_way(thing):
file_data = f"""This is the entire template text from {thing.var1} about the time I got a
{thing.var2} stuck in my {thing.var3} at band camp."""
with open('output file.txt', 'w') as file:
file.write(file_data)

Answer 1

I'd use neither.
Using regex will be safer (ie you don't need to f-string and eval the entire file) and scalable (you don't need 30 calls to str.replace if you have 30 variables, just an entry in the mapping dict).
import re
table = {'<time>': '12:00',
'<date>': '1.1.1970'}
# imagine this being read from a file
string = '<time>, fixed text, <date>'
print(re.sub(r'(<.+?>)', lambda s: table.get(s.group(1), s.group(1)), string))
outputs
12:00, fixed text, 1.1.1970
Adapting to your case (where the values are attributes of an object)
All you have to do is use the object as the values for mapping dict.
...
thing = Thing('name', 'a', 'b', 'c')
table = {'<time>': thing.var1,
'<date>': thing.var2}
...
This can become cumbersome if you need to do something more complex (like if you have multiple objects) but of course it can be improved depending on your exact use-case.
For example, if the name of the placeholders coincide with the name of the attributes in the object you can just use vars as the mapping (don't forget to remove the < and > from the regex capturing group):
import re
class Thing:
def __init__(self, name, var1, var2, var3):
self.name = name
self.var1 = var1
self.var2 = var2
self.var3 = var3
thing = Thing('name', 'a', 'b', 'c')
string = '<var1>, fixed text, <var2>'
print(re.sub(r'<(.+?)>', lambda s: vars(thing).get(s.group(1), s.group(1)), string))
outputs
a, fixed text, b