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Dictionary

Each piece of information in a dictionary is stored as a key-value pair. When we provide a key, Python returns the value associated with that key.

Representation​

Use curly braces to define a dictionary. Use colons to connect keys and values, and use commas to separate individual key-value pairs.

Empty Dictionary - {}

Dictionary - {'Name': 'Deepak', 'Age':25}

Accessing Values in Dictionary​

Using square bracket notation to fetch data​

If we attempt to access a data item with a key, which is not part of the dictionary, we get KeyError

dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'}
print dict['Name']
print dict['Age']
Zara
7

Using built-in function dict.get() to fetch data from dictionary​

dict.get(key, default=None)

The methodget()returns a value for the given key. If key is not available then returns default value None. (or the provided default value)

dict = {'Name': 'Zabra', 'Age': 7}
print "Value : %s" %  dict.get('Age')
print "Value : %s" %  dict.get('Education')
print "Value : %s" %  dict.get('Education', "Never")
Value : 7
Value : None
Value : Never

For multi level dict hierarchies​

Returning a default empty dictionary will not raise AttributeError for second get()

sub_typology=project.json.get(**'area'**,{}).get(**'0'**,{}).get(**'areaSubTypology'**, **''**)

Updating in Dictionary​

dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'}
dict['Age'] = 8; # update existing entry

If key is not present, then an item is added in the dictionary

Adding in Dictionary​

dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'}
dict['School'] = "DPS School"; # Add new entry

Deleting in Dictionary​

dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'}
del dict['Name']; # remove entry with key 'Name'
dict.clear(); # remove all entries in dict
del dict ; # delete entire dictionary

If we attempt to delete a data item with a key, which is not part of the dictionary, we get KeyError

Length of a dictionary​

num_responses = len(fav_languages)

Properties of Dictionary Keys​

  • Dictionary values can be any arbitrary Python object, either standard objects or user-defined objects.
  • Duplicate key not allowed. When duplicate keys encountered during assignment, the last assignment wins.
dict = {'Name': 'Zara', 'Age': 7, 'Name': 'Manni'}
print "dict['Name']: ", dict['Name']

dict['Name']: Manni
  • Keys must be immutable. Otherwise "Type Error: objects are unhashable" error thrown. Strings, numbers, tuples can be used as dictionary keys

Built-in Dictionary functions & Methods​

Functions​

cmp(dict1, dict2)​

Compares elements of both dict

Not supported in python3, use dict1==dict2 instead

Library deepdiff can be used for advanced comparisons

- len(dict)​

Gives the total number of items in the dictionary

- str(dict)​

Produces a printable string representation of a dictionary

- type(variable)​

- Methods​

- dict.clear()​

Removes all elements of dictionary dict

- dict.copy()​

Returns a shallow copy of dictionary dict

  • dict.fromkeys(seq[, value])

Create a new dictionary with keys from seq and values set to value

seq− This is the list of values which would be used for dictionary keys preparation.

value− This is optional, if provided then value would be set to this value

seq = ('name', 'age', 'sex')
dict = dict.fromkeys(seq)
print "New Dictionary : %s" % str(dict)
New Dictionary : {'age': None, 'name': None, 'sex': None}

dict = dict.fromkeys(seq, 10)
print "New Dictionary : %s" % str(dict)
New Dictionary : {'age': 10, 'name': 10, 'sex': 10}
dict.get(key, default=None)

dict.has_key(key) (removed in python 3, use in operator instead)

dict = {'Name': 'Zabra', 'Age': 7}
'Name' in dict

True

'NoKey' in dict

False

- dict.items()​

The method items() returns a list of dict's (key, value) tuple pairs

dict = {'Name': 'Zara', 'Age': 7}
for key, value in dict.items():
 print (key, value)

Name Zara
Age 7

- dict.keys()​

looping through all keys

Others​

  • dict.setdefault(key, default=None)
  • dict.update(dict2)
  • dict.values()

Looping through all the values

Sorting a dictionary​

sorting by value​

print sorted(prices.items(), key = lambda x : x[1])

sorting by key​

print(sorted(prices.items())

Looping through all the keys in order​

for name in sorted(fav_language.keys()):
  print(name)

Merging two dictionaries​

# Python code to merge dict using update() method
def Merge(dict1, dict2):
    return(dict2.update(dict1))

# Driver code
dict1 = {'a': 10, 'b': 8}
dict2 = {'d': 6, 'c': 4}

# This return None
print(Merge(dict1, dict2))

# changes made in dict2
print(dict2)