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Python 3: An interactive deep dive

Dictionary Comprehensions

Explore how to create and use dictionary comprehensions in Python 3 to build dictionaries efficiently. Understand the syntax differences from list comprehensions, apply filtering, and see how to swap dictionary keys and values practically.

We'll cover the following...

A dictionary comprehension is like a list comprehension, but it constructs a dictionary instead of a list.

Python 3.5
import os, glob
metadata = [(f, os.stat(f)) for f in glob.glob('*test*.py')]             #①
print (metadata[0] )                                                     #②
# ('romantest3.py', os.stat_result(st_mode=33261, 
# st_ino=1057919, st_dev=2049, st_nlink=1, st_uid=1003, 
# st_gid=50, st_size=4640, st_atime=1472210494, 
# st_mtime=1472151083, st_ctime=1472210494))
metadata_dict = {f:os.stat(f) for f in glob.glob('*test*.py')}           #③
print (type(metadata_dict))                                              #④
#<class 'dict'>
print (list(metadata_dict.keys()))                                       #⑤
#['romantest2.py', 'romantest3.py', 'romantest1.py']
print (metadata_dict['romantest3.py'].st_size)                           #⑥
#4640

① This is not a dictionary comprehension; it’s a list comprehension. It finds all .py files with test in their name, then constructs a tuple of the filename and the file metadata (from calling the os.stat() function).

② Each item of the resulting list is a tuple.

③ This is a dictionary comprehension. The syntax is similar to a list ...