dict.update does not concat/replace if same key

[AvnishMidha]

BPO	32617
Nosy	@ericvsmith
Files	dict_try.py

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2018-01-22.07:46:05.737>
created_at = <Date 2018-01-22.06:22:13.019>
labels = ['interpreter-core', 'type-bug', 'invalid']
title = 'dict.update does not concat/replace if same key'
updated_at = <Date 2018-01-22.07:46:05.730>
user = 'https://bugs.python.org/AvnishMidha'

bugs.python.org fields:

activity = <Date 2018-01-22.07:46:05.730>
actor = 'eric.smith'
assignee = 'none'
closed = True
closed_date = <Date 2018-01-22.07:46:05.737>
closer = 'eric.smith'
components = ['Interpreter Core']
creation = <Date 2018-01-22.06:22:13.019>
creator = 'Avnish Midha'
dependencies = []
files = ['47400']
hgrepos = []
issue_num = 32617
keywords = []
message_count = 2.0
messages = ['310397', '310400']
nosy_count = 2.0
nosy_names = ['eric.smith', 'Avnish Midha']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue32617'
versions = ['Python 3.6']

[AvnishMidha]

The dict does not seem to work as one would expect a dictionary or a hashtable to work.

dict.update(otherdict) just replaces the entries as per otherdict, removing items which were extra in dict too and just keeping items as per new data in otherdict only. Shouldn't this be actually about updating the dict with new/updated entries and not touching those entries which pre-existed?

Sample program below:
import pandas as pd

x = {'name': ['A', 'B', 'C'], 'col2': [3, 4,6]}
y = {'name': ['B', 'C', 'D', 'E'], 'col2': [1,2, 9, 10]}

print ("X = \n" + str(x))

print ("Y = \n" + str(y))

x.update(y)

print ("updated dict = \n" + str(x) + "\n")

Output:

X = 
{'name': ['A', 'B', 'C'], 'col2': [3, 4, 6]}
Y = 
{'name': ['B', 'C', 'D', 'E'], 'col2': [1, 2, 9, 10]}
updated dict = 
{'name': ['B', 'C', 'D', 'E'], 'col2': [1, 2, 9, 10]}

Expected value:
updated dict =
{'name': ['A','B', 'C', 'D', 'E'], 'col2': [3, 1, 2, 9, 10]}

Somehow to do this basic hashtable or dictionary update kind of operation is too complex in python and update() should have actually functioned as expected above IMO.

[ericvsmith]

I'm sorry the name is confusing to you, but this behavior is correct, and how dict.update is defined:

https://docs.python.org/3.6/library/stdtypes.html#dict.update

As such, it cannot be changed.

If you want a function that merges values, you'll need to write it yourself.

Eric.