练习：赋值和修改变量
现在该你来使用变量了。这道练习的注释（以 # 开头的行）提供了创建和修改变量的说明。请在每条注释后面根据说明写一行代码。
注意，这段代码使用了科学记数法来定义很大的数字。4.445e8 等于 4.445 * 10 ** 8，也就是 444500000.0。
# The current volume of a water reservoir (in cubic metres)
reservoir_volume = 4.445e8
# The amount of rainfall from a storm (in cubic metres)
rainfall = 5e6
# decrease the rainfall variable by 10% to account for runoff
rainfall = 5e6*0.9
# add the rainfall variable to the reservoir_volume variable
reservoir_volume = reservoir_volume + rainfall
# increase reservoir_volume by 5% to account for stormwater that flows
# into the reservoir in the days following the storm
reservoir_volume = reservoir_volume*1.05
# decrease reservoir_volume by 5% to account for evaporation
reservoir_volume = reservoir_volume*0.95
# subtract 2.5e5 cubic metres from reservoir_volume to account for water
# that's piped to arid regions.
reservoir_volume = reservoir_volume - 2.5e5
# print the new value of the reservoir_volume variable
print(reservoir_volume)
更改变量
更改变量会如何影响到用该变量定义的另一个变量？我们来看一个示例。
这是关于山景城人口和人口密度的原始数据。
>>> mv_population = 74728
>>> mv_area = 11.995
>>> mv_density = mv_population/mv_area
现在我们重新定义 mv_population 变量：
（注意：后续代码跟在上面三行代码后面，而非重新开始）
>>> mv_population = 75000
习题 2/3
思考一下上方的代码，下面的表达式输出会是什么？
>>> print(int(mv_density))
<input type="radio" readonly="" class="index-module--radio--J18uO" id="ureact-radio-bpz7dzls" value="on" style="box-sizing: border-box; margin: 0px 0.5em 0px 0px; font-family: inherit; font-size: inherit; line-height: inherit; overflow: visible; padding: 0px; left: -999em; outline: none; position: absolute;"> <label for="ureact-radio-bpz7dzls" class="index-module--label--2OxfH" style="box-sizing: inherit; display: flex; font-weight: 400; user-select: none; align-items: center; color: rgb(46, 61, 73); font-size: 14px; padding-left: 1.5em; vertical-align: middle; cursor: pointer;">6252.60525219</label>
<input type="radio" readonly="" class="index-module--radio--J18uO" id="ureact-radio-3i2z60yj" value="on" style="box-sizing: border-box; margin: 0px 0.5em 0px 0px; font-family: inherit; font-size: inherit; line-height: inherit; overflow: visible; padding: 0px; left: -999em; outline: none; position: absolute;"> <label for="ureact-radio-3i2z60yj" class="index-module--label--2OxfH" style="box-sizing: inherit; display: flex; font-weight: 400; user-select: none; align-items: center; color: rgb(46, 61, 73); font-size: 14px; padding-left: 1.5em; vertical-align: middle; cursor: pointer;">6229.92913714</label>
<input type="radio" readonly="" class="index-module--radio--J18uO" id="ureact-radio-ldqbn5y7" value="on" style="box-sizing: border-box; margin: 0px 0.5em 0px 0px; font-family: inherit; font-size: inherit; line-height: inherit; overflow: visible; padding: 0px; left: -999em; outline: none; position: absolute;"> <label for="ureact-radio-ldqbn5y7" class="index-module--label--2OxfH" style="box-sizing: inherit; display: flex; font-weight: 400; user-select: none; align-items: center; color: rgb(46, 61, 73); font-size: 14px; padding-left: 1.5em; vertical-align: middle; cursor: pointer;">6229</label>
6246.92823723
<input type="radio" readonly="" class="index-module--radio--J18uO" id="ureact-radio-4i3xr4a3" value="on" style="box-sizing: border-box; margin: 0px 0.5em 0px 0px; font-family: inherit; font-size: inherit; line-height: inherit; overflow: visible; padding: 0px; left: -999em; outline: none; position: absolute;"> <label for="ureact-radio-4i3xr4a3" class="index-module--label--2OxfH" style="box-sizing: inherit; display: flex; font-weight: 400; user-select: none; align-items: center; color: rgb(46, 61, 73); font-size: 14px; padding-left: 1.5em; vertical-align: middle; cursor: pointer;">以上都不对。</label>
习题 3/3
这是美国的州列表，按照加入联邦的日期排序。假设你想为特拉华州创建一个变量并赋一个值，表示它首先加入了联邦。以下哪些项是有效的 Python 变量名和赋值？
<input type="checkbox" readonly="" class="index-module--checkbox--3fDuX" id="ureact-checkbox-00b5byoq" value="on" style="box-sizing: border-box; margin: 0px 0.5em 0px 0px; font-family: inherit; font-size: inherit; line-height: inherit; overflow: visible; padding: 0px; flex: 0 0 auto; height: 16px; outline: none; width: 16px; cursor: pointer;"> <label for="ureact-checkbox-00b5byoq" class="index-module--label--2YzyD" style="box-sizing: inherit; display: inline-block; font-weight: 400; user-select: none; font-size: 14px; margin-left: 0.5em; cursor: pointer;">
del = 1</label>delaware = 1<input type="checkbox" readonly="" class="index-module--checkbox--3fDuX" id="ureact-checkbox-1lywko7c" value="on" style="box-sizing: border-box; margin: 0px 0.5em 0px 0px; font-family: inherit; font-size: inherit; line-height: inherit; overflow: visible; padding: 0px; flex: 0 0 auto; height: 16px; outline: none; width: 16px; cursor: pointer;"> <label for="ureact-checkbox-1lywko7c" class="index-module--label--2YzyD" style="box-sizing: inherit; display: inline-block; font-weight: 400; user-select: none; font-size: 14px; margin-left: 0.5em; cursor: pointer;">
1 de = first</label>de = 1
