Meaning of exclamation mark in Python parser of ArcGIS Field Calculator?

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In the following line, I don't understand what those exclamation marks mean? And why put everything in quotation? I don't understand the purpose of the variable expression, please explain to me as I'm new to python

expression = "crossing(!CROSS_ROW!,!ROW_SCORE!)" codeblock = """def crossing(crosses, row_score): if crosses != 0: return 5 else: return row_score""" The code gives a score to pipes (from pipe layer) that cross, or passes next to roads (from road layer) depending on the width of the road. this was the code-block that assigned a score of 5 if the pipe crosses.



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