python changing value of column with loc doesnt change nothing

I have df that I want to change the column ‘b’ for all rows when column a is equal to 1, to numbers in the range [0.5,1].
for example:

a   b
0   0.2
0   0.4
1   0.02
1   0.001

desire df:

a   b
0   0.2
0   0.4
1   0.7
1   0.8

My code is:

df[df['a']==1].reset_index(drop=True).loc[0:len(df[df['a']==1]), 'b'] = np.linspace(0.5,0.99,len (df[df['a']==1]))

But nothing changed.
Thx

>Solution :

IIUC use:

m = df['a']==1
df.loc[m, 'b'] = np.linspace(0.5,0.99, m.sum())
print (df)
   a     b
0  0  0.20
1  0  0.40
2  1  0.50
3  1  0.99