python dictionary tips and tricks

Here ar some dict methods, tips and trick which I find useful

The dict.clear method vs dict.new

d = {"a": "1"}
d2 = d
d = {}             # --> creates new dict into d
print (d2)         # --> outputs: {"a": "1"}
d = {"a": "1"}
d2 = d             # --> d2 is a referance to d
d.clear()          #  so now both d and d2  are cleared
print (d2)         # --> outputs: {}

Aassigning d = {} creates a new, empty dictionary. This leaves d2 pointing at the old dictionary. so it contains the items of the original dictionary. However, using d.clear() clears the same dictionary that d and d2 both point at.

Convert two lists into a dictionary - using zip

keys = ['a', 'b', 'c']
values = [1, 2, 3]
if len(keys)==len(values):
        my_dict = dict(zip(keys, values))
        print(my_dict) # --> {'a': 1, 'b': 2, 'c': 3}

Be careful! - you must check that the length of key and values are the same, or else:

keys = ['a']
values = [1, 2, 3]
my_dict = dict(zip(keys, values))
print(my_dict) # --> {'a': 1}

keys = ['a', 'b', 'c']
values = [1]
my_dict = dict(zip(keys, values))
print(my_dict) # --> {'a': 1}

Convert a dictionary into tuples of keys, and values using zip

This is less common use - an yet here it is:

new_keys,new_values= zip( *my_dict.items() )
print (type(new_keys))                       # --> <class 'tuple'>
print (new_keys)                             # --> ('a', 'b', 'c')
print (new_values)                           # --> (1, 2, 3)

Convert a List to Dictionary with same values

My preferred method:

keys = ['a', 'b', 'c']
my dict = dict.fromkeys(keys , 1)

More pythonic way:

keys = ['a', 'b', 'c']
my_dict = { k : 'same_value' for k in keys }

Assignment (=) vs dict.copy

Take a look here:

d = {"a": "1"}
        
d["b"]=2
print (d,d2)           # --> outputs: {'a': '1', 'b': 2} {'a': '1', 'b': 2}

d = {"a": "1"}
d2 = d.copy()
d["b"]=2
print (d,d2)           # --> outputs: {'a': '1', 'b': 2} {'a': '1'}

The assignment operation (=) in the line: d2 = d creates reference from d2 which points to d. The method dict.copy creates exact copy of the dictionary d.

Set\Get value if not exists for a specific key

Set… use setdefault(<key>, <defualt>)

This is so useful - I and always forget to use it… Get the value of the ‘species’ item…

• If the ‘species’ item does not exist, insert ‘species’ with the value Iris-setosa’.
• Else keep the current value

print (iris) # --> {
             #      'sepal_length': 1, 
             #      'sepal_width': 5.1, 
             #      'petal_length': 3.5, 
             #      'petal_width': 1.4
             #      }
             
iris.setdefault("species", "Iris-setosa")

print (iris) # --> {
             #      'sepal_length': 1, 
             #      'sepal_width': 5.1, 
             #      'petal_length': 3.5, 
             #      'petal_width': 1.4
             #      'species': 'Iris-setosa'        <-- new key:value pair
             #       }

iris.setdefault("species","Iris-versicolor")

print (iris["species"]) # --> Iris-setosa           <-- the old value "Iris-setosa" did not chaneged

Get… use get(<key>, <defualt>)

d = {'a': 1, 'b': 2, 'c': 3}

print(d.get('b'))      # --> 20
print(d.get('z'))      # --> None
print(d.get('z',777))  # -->777

BTW dict.get('my_none_exists_key') returns None, but d['my_none_exists_key'] will raise an excetion:

## Merging dictionaries
Here we try to merge two dictionaries which has different value for the key 'b':

```python
d1 = { 'a': 1, 'b': 2 }
d2 = {         'b': 3, 'c': 4 }

merged = { **d2, **d1 }
print (merged)                   # --> {'a': 1, 'b': 3, 'c': 4}    <-- d['b']=3
merged = { **d2, **d1 }
print (merged)                   # --> {'b': 2, 'c': 4, 'a': 1}    <-- d['b']=4 

merged = { **d1, **d2}
print (sorted(merged.items()))     # -->  [('a', 1), ('b', 3), ('c', 4)] <-- d['b'] =3            
merged = { **d2, **d1 }
print (sorted(merged.items()))     # -->  [('a', 1), ('b', 2), ('c', 4)] <-- d['b'] =2            

The creation of the merged dictionary {d1,d2,...dn} add d2 after d1 and updates the values of a merged dictionary by the later dictionaries values. In the first case d2['b'] will overide ```d1[‘b’]``