Python: Dictionaries

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Dictionaries are unordered collections.
1. Accessed by key, not offset position
2. Unordered collections of arbitrary objects
3. Variable-length, heterogeneous, and arbitrarily nestable
4. Of the category "mutable mapping"
5. Tables of object references (hash tables)

you can use dir(dict) and help(dict) to learn more.
Examples:

D={}
D={'name': 'Bob, 'age', 40}
E={ 'CTO' : D} # nesting
D=dict(name='bob', age=40)
D=dict( [('name','bob'), ('age',40)]) # key value pairs
D=dict(zip(keylist, valuelist)) # key value lists
D=dict.fromkeys(['name', 'age']) # set up keys first, value are None
D['name']
E['CTO']['age']
'age' in D
D.keys()   # list of keys
D.values()
D.items() # list of tuples, evey tuple is a key value pair
D.copy()  # copy
D.clear()
D.update(D2)  #merge by keys
D.get(key, default?)
D.pop(key, default?)
D.setdefault(key,default?) # fetch by key, if absent set default (or None)
D.popitem()
len(D)  #lenth: number of stored entries
D[key]=42
del D[key]
list(D.keys())
D1.keys() & D2.keys()
D.viewkeys(), D.viewvalues()
D={x: x*2 for x in range(10) }  # dictionary comprehension

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关键词：python Aries CTI dic TIO references category tables learn

more on dictionaries

fetching nonexistent keys is normally error a['nonexist'], but the get method returns a default value - None.
D.get('nonexist')

the update() is similar to concatenation for dictionaries

[key for (key,value) in table.items() if value=v] # for Value->Key

[key for key in table.keys() if table[key]=v]  # ditto

sequence operations do not work.

assigning to new indexes adds entries

keys need not always be stringsm[(2,3,4)]=88 # tuple keys. sparse data structure

dict(zip(['a','b','c'], [1,2,3]))  # make a dict from zip result

D={k: v for (k,v) in zip(['a','b','c'], [1,2,3])