A comment begins with the # sign and can appear anywhere (outside of strings). Everything from the # to the end of the line is ignored by the Python interpreter. Commonly used for human-readable notes.
sum = 2 + 2 # this is a statement name = raw_input("What is your name?") # these are two statements print "Hello,", name print "Did you know that your name has", \ len(name), "letters?" # This is one statement spread across 2 lines # Another way to extend a statement across several lines print "Here is your name repeated 7 times:", ( name * 7 )The Python interpreter will start at the top of the file and make sure the program is valid Python. (If not it will give an error message.) After that it executes each statement in order, starting from the top. It stops when it reaches the end of the file or if there is an uncaught exception.
EcoRI = "GAATTC" sequence = raw_input("Enter a DNA sequence:") if EcoRI in sequence: print "Sequence contains an EcoRI site" # This is a one-line block import sys sequence2 = raw_input("Enter another sequence:") if len(sequence2) < 100: print "Sequence is too small. Throw it back." # a two-line block sys.exit(0) sequences = (sequence, sequence2) for seq in sequences: print "sequence length =", len(seq) # a block ... for c in "ATCG": print "#%s = %d" % (c, seq.count("C")) # ... with a block inside it
>>> if 5 > 4: ... print "5 is larger" ... 5 is larger >>>If the expression is false and there is an else: block, execute those statements instead.
>>> if 4 > 5: ... print "4 is larger" ... else: ... print "5 is larger" ... 5 is larger >>>
This page based on Perl Statements