In [1]:

    

'A'


    

    
Out[1]:





'A'

In [2]:

    

'ACGT'


    

    
Out[2]:





'ACGT'

In [3]:

    

st = 'ACGT'


    

In [4]:

    

len(st) # getting the length of a string


    

    
Out[4]:





4

In [5]:

    

'' # empty string (epsilon)


    

    
Out[5]:





''

In [6]:

    

len('')


    

    
Out[6]:





0

In [7]:

    

import random
random.choice('ACGT') # generating a random nucleotide


    

    
Out[7]:





'C'

In [8]:

    

random.choice('ACGT') # repeated invocations might yield different nucleotides


    

    
Out[8]:





'C'

In [9]:

    

random.choice('ACGT') # repeated invocations might yield different nucleotides


    

    
Out[9]:





'T'

In [10]:

    

random.choice('ACGT') # repeated invocations might yield different nucleotides


    

    
Out[10]:





'A'

In [11]:

    

random.choice('ACGT') # repeated invocations might yield different nucleotides


    

    
Out[11]:





'G'

In [12]:

    

# now I'll make a random nucleotide string by concatenating random nucleotides
st = ''.join([random.choice('ACGT') for _ in range(40)])
st


    

    
Out[12]:





'GTCACATAATGCTGTAGATTAAGCGAGAAGACCTTAGGTC'

In [13]:

    

st[1:3] # substring, starting at position 1 and extending up to but not including position 3
# note that the first position is numbered 0


    

    
Out[13]:





'TC'

In [14]:

    

st[0:3] # prefix of length 3


    

    
Out[14]:





'GTC'

In [15]:

    

st[:3] # another way of getting the prefix of length 3


    

    
Out[15]:





'GTC'

In [16]:

    

st[len(st)-3:len(st)] # suffix of length 3


    

    
Out[16]:





'GTC'

In [17]:

    

st[-3:] # another way of getting the suffix of length 3


    

    
Out[17]:





'GTC'

In [18]:

    

st1, st2 = 'CAT', 'ATAC'


    

In [19]:

    

st1


    

    
Out[19]:





'CAT'

In [20]:

    

st2


    

    
Out[20]:





'ATAC'

In [21]:

    

st1 + st2 # concatenation of 2 strings


    

    
Out[21]:





'CATATAC'