Replacing strings with variables inside file in Python

Question

I have a bunch of files with many tags inside of the form {my_var}, {some_var}, etc. I am looking to open them, and replace them with my_var and some_var that I've read into Python.

To do these sorts of things I've been using inspect.cleandoc():

import inspect, markdown
my_var='this'
some_var='that'
something=inspect.cleandoc(f'''
    All my vars are {some_var} and {my_var}. This is all.
''')
print(something)
#All my vars are that and this. This is all.

But I'd like to do this by reading files file1.md and file2.md

### file1.md
There are some strings such as {my_var} and {some_var}.
Done.

### file2.md
Here there are also some vars: {some_var}, {my_var}. Also done.

Here's the Python code:

import inspect, markdown
my_var='this'
some_var='that'

def filein(file):
    with open(file, 'r') as file:
        data = file.read()
    return data

for filei in ['file1.md','file2.md']:
    fin=filein(file)
    pre=inspect.cleandoc(f'''{fin}''')

However, the above does not evaluate the strings inside filei and replace them with this (my_var) and that (some_var), and instead keeps them as strings {my_var} and {some_var}.

What am I doing wrong?

Answer 1

You can use the .format method.

You can use ** to pass it a dictionary containing the variable.

Therefore you can use the locals() or globals(), which are dictionary of all the locals and globals variables.

e.g.

text = text.format(**globals())

Complete code:

my_var="this"
some_var="that"

for file in ["file1.md", "file2.md"]:
    with open(file, "r") as f:
        text = f.read()
    text = text.format(**globals())
    print(text)

Answer 2

• f-strings are a static replacement mechanism, they're an intrinsic part of the bytecode, not a general-purpose templating mechanism
• I've no idea what you think inspect.cleandoc does, but it does not do that.
• Python generally avoids magic, meaning it really doesn't give a rat's ass about your local variables unless you specifically make it, which is not the case here. Python generally works with explicitely provided dicts (mappings of some term to its replacement).

I guess what you want here is the format/format_map methods, which do apply to format strings using {} e.g.

filein(file).format(my_var=my_var, some_var=some_var)

This can be risky if the files you're reading are under the control of a third party though: str.format allows attribute access and thus ultimately provides tools for arbitrary code execution. In that case, tools like string.Template, old-style string substitution (%) or a proper template engine might be a better idea.