How can I iterate through a nested dictionary (withnin a nested list) and copy the values into a new list

You need to follow the structure you have:

• iterate over the match element from the outer array
• for each array iterate over the players from the array given by key 'players'
• for each player check name, and retrieve name if it is the player you're looking for.

def get_won(data, name):
    result = []
    for match in data:
        for player in match['players']:
            if player['player_name'] == name:
                result.append(player['won'])
  return result

print(get_won(game_data, 'Tom'))  # [True, False]
print(get_won(game_data, 'Fred')) # [True]
print(get_won(game_data, 'Anna')) # [False]                                   

List-comprehension version

def get_won(data, name):
    return [player['won'] for match in data for player in match['players'] 
                                            if player['player_name'] == name]

Here's a way to do that with list comprehension:

all_players = [p for m in game_data for p in m['players']]
tom = [m["won"] for m in all_players if m['player_name'] == "Tom"]

Results in [True, False]

Here you go:

res = []
for game in game_data:
    for player in game['players']:
        if player['player_name'] == 'Tom':
            res.append(player['won'])
print(res)

Output:

[True, False]

Try this:

game_data = [{"match_id":1, "players":[{"player_name":"Tom", "won":True},
                                       {"player_name":"Anna", "won":False}]}, 
             {"match_id":2, "players":[{"player_name":"Tom", "won":False},
                                       {"player_name":"Fred", "won":True}]}]


tom_won = []

for each_match in game_data:
    for each_player in each_match["players"]:
        player_name = each_player["player_name"]
        if player_name == "Tom":
            tom_won.append(each_player["won"])

Also, you can get all players' result by using defaultdict:

game_data = [{"match_id":1, "players":[{"player_name":"Tom", "won":True},
                                       {"player_name":"Anna", "won":False}]}, 
             {"match_id":2, "players":[{"player_name":"Tom", "won":False},
                                       {"player_name":"Fred", "won":True}]}]


import collections

player_won = collections.defaultdict(list)
for each_match in game_data:
    for each_player in each_match["players"]:
        player_name = each_player["player_name"]
        player_won[player_name].append(each_player["won"])

I'm using python2.7, but the code should be easily changed to run in python3.

I would suggest working on the data structure first, for example:

game_data = [
    {"match_id": 1, "results": {"Tom": True, "Anna": False}}, 
    {"match_id": 2, "results": {"Tom": False, "Fred": True}}
]

Which you can then use to produce the summary for one player:

>>> [match['results']['Tom'] for match in game_data]
[True, False]

This is not a really good data structure yet but at least it's easier to work with.

What could also make sense is to add a bit of coherence between the won:

game_data = [
    {"match_id": 1, "winner": 1, "player_names": ["Anna", "Tom"]}, 
    {"match_id": 2, "winner": 0, "player_names": ["Fred", "Tom"]}, 
]

Then:

def results(match, name):
    return match['player_names'][match['winner']] == name

player_results = [results(match, 'Tom') for match in game_data]

To avoid duplication of names you could probably use identifiers for players too:

from collections import namedtuple

Game = namedtuple('Game', ['match_id', 'winner', 'players'])

game_data = [
    Game(match_id=1, winner=1, players=[0, 1]),
    Game(match_id=2, winner=2, players=[1, 2]),
]

player_id = {name: i for i, name in enumerate(['Anna', 'Tom', 'Fred'])}


player_results = [game.winner == player_id['Tom'] for game in game_data]

You are the only one that can really answer the question "what is the best data structure" but I hope this will help a bit.

Also note that you can simply filter results to only show matches in which the player participated, for example with the last data structure:

[game.winner == player_id['Fred'] for game in game_data if player_id['Fred'] in game.players]

Which is not the cleanest form, but you could have an iterator to first get the games in which a player participated and then get it's results:

def player_games(games, name):
    return (game for game in game_data if player_id[name] in game.players)

[game.winner == player_id['Fred'] for game in player_games(game_data, 'Fred')]