There are a couple of ways:

pathlib.Path().rglob()

Use pathlib.Path().rglob() from the pathlib module, which was introduced in Python 3.5.

from pathlib import Path

for path in Path('src').rglob('*.c'):
    print(path.name)

glob.glob()

If you don't want to use pathlib, use glob.glob():

from glob import glob

for filename in glob('src/**/*.c', recursive=True):
    print(filename)   

For cases where matching files beginning with a dot (.); like files in the current directory or hidden files on Unix based system, use the os.walk() solution below.

os.walk()

For older Python versions, use os.walk() to recursively walk a directory and fnmatch.filter() to match against a simple expression:

import fnmatch
import os

matches = []
for root, dirnames, filenames in os.walk('src'):
    for filename in fnmatch.filter(filenames, '*.c'):
        matches.append(os.path.join(root, filename))

This version should also be faster depending on how many files you have, as the pathlib module has a bit of overhead over os.walk().

I think you have misunderstood the meaning of recursive=True. It is used to allow ** to mean searching subdirectories to a variable depth -- glob(folder+'\\**\\*.ext',recursive=True) will find any *.ext files in the starting folder or anywhere below it. Without recursive=True, ** would be interpreted in the normal way, with each * meaning to match any number of characters excluding \, and ** would therefore just be equivalent to * -- in other words glob(folder+'\\**\\*.ext',recursive=False) would search for any *.ext files that are exactly one level of subdirectory down from the starting folder, but not in the starting folder itself or in deeper subdirectories.

Exactly which you should use will depend on what you are trying to look for -- and it may indeed be that you need to select between different cases with an if statement -- but there is no use case for using ** with recursive=False, because it is just equivalent to * in that case.

The most likely useful options are:

glob(folder+'\\*.ext',recursive=False)  # search in top folder only
glob(folder+'\\*\\*.ext',recursive=False)  # search exactly one level down
glob(folder+'\\**\\*.ext',recursive=True)  # search to arbitrary depth

The ,recursive=False can be omitted because it is the default.

If there were exactly two options that you wanted to select between using a boolean option, you could always write your own wrapper function, for example:

def myglob(folder, pattern, subdirs=False):
    if subdirs:
        return glob(f'{folder}\\**\\{pattern}', recursive=True)
    else
        return glob(f'{folder}\\{pattern}')

print(myglob(folder, '*.ext', subdirs=False))
print(myglob(folder, '*.ext', subdirs=True))
print(myglob(folder, '*.ext'))  # same as subdirs=False in this example

(In Unix-like operating systems, read / for \ above.)

Use **/* as a pattern:

>>> from pprint import pprint as pp
>>> import pathlib as pl
>>>
>>>
>>> p = pl.Path(".")
>>>
>>> old_way = list(p.glob("**/"))  # Your way
>>> pp(old_way)
[WindowsPath('.'),
 WindowsPath('dir0'),
 WindowsPath('dir1'),
 WindowsPath('dir1/dir10')]
>>>
>>> new_way = list(p.glob("**/*"))
>>> pp(new_way)
[WindowsPath('dir0'),
 WindowsPath('dir1'),
 WindowsPath('file0.txt'),
 WindowsPath('dir0/file00.txt'),
 WindowsPath('dir1/dir10'),
 WindowsPath('dir1/file10.txt')]
>>>
>>> newer_way = [p] + list(p.glob("**/*"))  # Prepend globed dir
>>> pp(newer_way)
[WindowsPath('.'),
 WindowsPath('dir0'),
 WindowsPath('dir1'),
 WindowsPath('file0.txt'),
 WindowsPath('dir0/file00.txt'),
 WindowsPath('dir1/dir10'),
 WindowsPath('dir1/file10.txt')]

Here's [Python.Docs]: pathlib - Path.glob(pattern) for reference.