Convert string to dictionary in a dataframe

I have a dataframe that looks like this

df = pd.DataFrame({'col_1': ['1', '2', '3', '4'],
                   'col_2': ['a:b,c:d', ':v', 'w:,x:y', 'f:g,h:i,j:']
                   })

Datatype of col_2 is currently string. I want to extract the first key and first value from col_2 as col_3 and col_4 respectively. So the output should look like

pd.DataFrame({'col_1': ['a', 'b', 'c', 'd'],
              'col_2': ['a:b,c:d', ':v', 'w:,x:y', 'f:g,h:i,j:'],
              'col_3': ['a','','w','f'],
              'col_4': ['b','v','','g']
               })

Here is what i have done so far is this

df['col_3'] = df['col_2'].apply(lambda x: x.split(":")[0])
df['col_4'] = df['col_2'].apply(lambda x: x.split(":")[1])

But this obviously doesn't work because its not a dictionary.

This is a good job for a regex and str.extract:

df[['col_3', 'col_4']] = df['col_2'].str.extract(r'^([^:,]*):([^:,]*)')

Output:

  col_1       col_2 col_3 col_4
0     1     a:b,c:d     a     b
1     2          :v           v
2     3      w:,x:y     w      
3     4  f:g,h:i,j:     f     g

regex demo

Another option with string methods:

df[["col_3", "col_4"]] = df["col_2"].str.split(",", n=1).str[0].str.split(":", expand=True)

Result:

  col_1       col_2 col_3 col_4
0     1     a:b,c:d     a     b
1     2          :v           v
2     3      w:,x:y     w      
3     4  f:g,h:i,j:     f     g