Computing starts to become interesting when operators and types combine to form expressions . Throughout science, there are expressions about equalities or inequalities, typically called formulas or equations. Scientific breakthroughs sometimes have concise technical statements expressed by such formulas. Students learn how to calculate with expressions (usually algebraic expressions), but the same general way of manipulating symbols happens with logic, balancing chemical reaction equations, deducing voltages for a circuit, and so on. Every student learns how, with the aid of a calculator, to find the x satisfying x = 8/(52 − 3× 7). Early on, students learn rules of what to do first, how to proceed from one step to the next, in order to solve such problems. Similarly, programming languages need syntax rules that guide interpretation of source code, which is the topic of this chapter. A Python program might contain an expression

not True or not False and True

Without syntax rules , this expression seems ambiguous. Programs can also mix operators and arguments having a variety of types; here, syntax rules (like grammar in English) can improve programming style, so that programs written by one person are more easily understood by someone else.